Mri scanner with active interference suppression and interference suppression method for an mri scanner

ABSTRACT

The invention relates to an MRI scanner and a method for operation of said MRI scanner. The MRI scanner has a first receiving antenna for receiving a magnetic resonance signal from a patient in a patient tunnel, a second receiving antenna for receiving a signal having the Larmor frequency of the magnetic resonance signal, and a receiver. The second receiving antenna is located outside of the patient tunnel or near an opening thereof. The receiver has a signal connection to the first receiving antenna and the second receiving antenna and is designed to suppress an interference signal by the second receiving antenna in the magnetic resonance signal received by the first receiving antenna.

The invention relates to an active interference suppression method in anMRI scanner and also to an MRI scanner with a receiver.

MRI scanners are imaging facilities, which for imaging an examinationobject align nuclear spins of the examination object with a strongexternal magnetic field and excite them with a magnetic alternatingfield for precession about this alignment. The precession or return ofthe spins from this excited state back into a state with lower energy inits turn creates as a response a magnetic alternating field, which isreceived via antennas.

A spatial encoding is impressed on the signals with the aid of magneticgradient fields, which subsequently makes it possible to assign thereceived signal to a volume element. The received signal is thenevaluated and a three-dimensional imaging representation of theexamination object is provided. To receive the signal, local receivingantennas, known as local coils, are preferably used that, to achieve abetter signal-to-noise ratio, are arranged directly on the examinationobject. The receiving antennas can also be built into a patient couch.

MRI scanners require radio frequency screening in two respects. On theone hand radio frequency pulses with powers in the kilowatt range aregenerated to excite the nuclear spins, which are only partly absorbed inthe patient. Radio waves that leave the patient tunnel are radiated outinto the room and must therefore be screened out to adhere to emissionlimit values.

Conversely the magnetic resonance signals to be received for the imagingare extremely weak. In order to obtain a sufficient signal-to-noiseratio (SNR) here, a screening out of external interference signals isrequired.

In the prior art complex screening cabins are therefore installed aroundan MRI scanner, in order to reduce both emissions and also immissions.

A knee coil with local screening is known from publication DE 10 2014207 843.

An object of the invention could therefore be to reduce the outlay forscreening.

The object is achieved by an inventive MRI scanner as claimed in claim 1and claim 2 and also by an inventive method for operation of the MRIscanner as claimed in claim 21 and claim 22.

The inventive MRI scanner has a patient tunnel, a first receivingantenna for receiving a magnetic resonance signal from a patient,wherein the patient is in the patient tunnel, a second receiving antennafor receiving a signal having the Larmor frequency of the magneticresonance signal and a receiver. The receiver has a signal connection tothe first receiving antenna and the second receiving antenna and ispreferably designed to prepare the magnetic resonance signal forimaging.

The second receiving antenna is arranged outside or in the vicinity ofan opening of the patient tunnel. An opening of the patient tunnel isseen in particular as the openings through which the patient couch withthe patient is moved into the patient tunnel and, depending on theregion to be examined, also leave the patient tunnel again at theopposite end. In the vicinity in this case is seen as a distance fromthe opening of less than 0.1 m, 0.2 m. 0.5 m, 1 m or 2 m. In thevicinity can also be seen as a distance from the opening of less than aquarter wavelength or a half wavelength of a radio wave in the airhaving a Larmor frequency of the MRI scanner.

The first receiving antenna in this case preferably receives themagnetic resonance signal, which however always has a small portion ofthe interference signal. Conversely the second receiving antenna notonly receives the interference signal, but also a minimal or negligibleportion of the magnetic resonance signal. For the sake of simplicity thesignal received by the first receiving antenna will still be referred tobelow as the magnetic resonance signal (even when having a portion ofthe interference signal to be removed) and the signal received by thesecond receiving antenna will be referred to as the interference signal.

The receiver is designed in this case to suppress an interference signalreceived with the second receiving antenna in a magnetic resonancesignal received by the first receiving antenna. Examples of forms ofembodiment are described in greater detail for the dependent claims.

In the form of embodiment of claim 2, in this case the possibility isconsidered in particular that the interference signal is more widebandthan the magnetic resonance signal. The portions outside the frequencyrange of the magnetic resonance signal correlate in this case with theportions within the frequency range of the magnetic resonance signal.Therefore it can also be sufficient in accordance with the invention forthe second receiving antenna to receive the wideband interference signaland for the receiver to be designed to evaluate this signal only partly,for example in a frequency range that is not the same as or is outsidethe frequency range of the magnetic resonance signal and then tosuppress the interference signal in a magnetic resonance signal receivedby the first receiving antenna as a function of this part signal. As analternative or in addition it would also be conceivable however for thesecond receiving antenna already only to receive frequencies outside thefrequency range of the magnetic resonance signal and to forward these tothe receiver. As a result of the correlation for example the amplitudeof the frequency components of the interference signal in the frequencyrange of the magnetic resonance signal can be linked with an amplitudeoutside it. In this way it is also conceivable for the second receivingantenna, in conjunction with receiver during a magnetic resonance scan,only to monitor frequency components outside the frequency range of themagnetic resonance signal for interference signals and to suppress theinterference signal in the signals of the first receiving antenna. Itwould be possible in this case for example for the receiver to establisha relationship between the interference signal from the first receivingantenna and interference signal portion from the second receivingantenna, also referred to as the transfer function, for an interferencesignal in the frequency range of the magnetic resonance signals betweendetections of magnetic resonance signals and during the detection ofmagnetic resonance signals for example, to adapt the amplitude orscaling on the basis of portions of the interference signal received bythe second receiving antenna outside the frequency range of the magneticresonance signal.

It is also conceivable for a filter to be provided between the secondreceiving antenna and the receiver, which preferably lets theinterference signal through and suppresses magnetic resonance signals.For example a filter that suppresses the magnetic resonance signalscould be provided for an interference element with portions outside thefrequency band of the magnetic resonance signal. The filter could alsobe adaptive or controllable. In this way it would be conceivable for theinterference suppression control to adapt the filter to the Larmorfrequency of a slice currently being acquired.

An explanation is given below as to how the interference signal can besuppressed in the signals of second receiving antenna by the receiver.For example the receiver can have a summation device, which forms alinear combination of one or more parameters dependent on the magneticresonance signal and interference signal. Furthermore the receiver canhave an interference suppression controller, which is designed to varythe parameter or parameters in such a way that an energy of theinterference signal is minimal in the linear combination. In this caseone or more parameters can be complex, in order to model a phasedisplacement or a phase displacement can be specified by a separateparameter. A number of parameters in particular allow the effectivesuppression of different interference sources.

Non-linear combinations of the signals depending on the parameter orparameters are also conceivable however.

The interference suppression controller could also weight interferencesignals with especially large amplitude especially heavily in theparameters compared to weaker interference signals in the interferencesuppression, since through the especially great distance of the signallevel in relation to a statistical background interference, these stronginterference signals are able to be suppressed especially well. In thiscase different interference signals are in particular seen asinterference signals that can be separated by different point of originby means of a number of second receiving antennas, occupy differentfrequency ranges or are differentiated by different temporal behavior.

The receiver can be designed in this case to carry out this inventiveprocessing of the received magnetic resonance signal and of theinterference signal for suppression of the interference signal in realtime, for example by means of a field-programmable gate array (FPGA) ora digital signal processor (DSP).

It is also possible however for the receiver to have a memory andinitially to store the received interference signal and the receivedmagnetic resonance signal, wherein the interference signal is onlysuppressed at a later point in time with a delay of for example theduration of an echo sequence, an excitation sequence or an entire imageacquisition of an individual slice or the entire image acquisitionsequence. The delay can for example be greater than 50 ms, 100 ms, 0.5s, 1 s, 10 s, 1 min or more.

A receiver in the sense of the invention can be seen as the hardware foranalog and/or digital radio frequency processing such as for exampleamplifiers, filters and mixers in real time, but also an imageevaluation unit for later generation of an image from the receivedmagnetic resonance signals.

The inventive method for operation of the inventive MRI scanner has thestep of receiving interference signals by means of the receiver via thesecond receiving antenna. Because of the arrangement of the secondantenna in the vicinity of the opening, the interference signal receivedby the second antenna received has scarcely any portions of the magneticresonance signal or none at all.

In another step the receiver receives a magnetic resonance signal viathe first receiving antenna. The first receiving antenna can involve abody coil or a local coil of the MRI scanner for example.

In a further step of the method, the receiver processes the magneticresonance signal as a function of the interference signal by thereceiver into a receive signal, wherein the dependency is a function ofa parameter. For example it is conceivable for the receiver to form alinear combination of interference signal and magnetic resonance signalwith the parameter as factor, wherein the parameter can be complex, inorder to map a phase displacement. A number of parameters are alsoconceivable.

In another form of embodiment of the inventive method, there isprovision for the receiver to receive portions of a widebandinterference signal outside the frequency range of the magneticresonance signal by means of the second receiving antenna. In this caseportions of the interference signal outside the frequency range of themagnetic resonance signal correlate with portions within the frequencyrange. For example it is conceivable for the amplitudes of theinterference signal within and outside the frequency range of themagnetic resonance signal to be proportional to one another, if the samesource is involved. In the suppression of the interference signal in themagnetic resonance signals received by the first receiving antennadescribed below, the scaling with the amplitude of the signal of thesecond receiving antenna can then be provided as the dependency forexample.

In an advantageous way, any influencing of the suppression of theinterference signal by the magnetic resonance signals during an imageacquisition can be avoided by the separate frequency ranges. In otherwords, by the evaluation of the signals received by the second receivingantenna by the receiver in frequency ranges that are not the same as thefrequencies of the magnetic resonance signals, it can be avoided thatscattered-in magnetic resonance signals are interpreted as aninterference signal can be avoided and suppressed by the interferencesuppression controller.

Preferably in this case the interference suppression controller alsotakes account of different characteristics of signals in the frequencyband of the magnetic resonance signals and outside it. These can forexample be different attenuations or signal delays, which can be takeninto account by other amplification factors and phase displacements. Thedifferent characteristics can be established for example by calibrationmeasurements to determine transfer functions, as will be explained belowfor the dependent claims.

In this case it is conceivable for interference signals to be detectedbetween and/or also during the MR signal acquisition by the receiver. Apermanent detection advantageously allows the temporal course of aninterference signal to be better detected and thus also better estimatedfor the future, which allows a more precise suppression.

An intermediate acquisition, i.e. between the acquisition of MR signals,makes it possible on the other hand to avoid the MR signals beingevaluated as interference signals. In this case it is also conceivablefor not only the second receiving antenna or antennas to detect theinterference signals in this case, but also simultaneously the firstreceiving antenna or first receiving antennas, so that it is knownexactly how an interference signal received by the second receivingantennas will be acquired by the first receiving antennas. Thisrelationship, also referred to as a transfer function, then allows amore precise determination of parameters for interference suppressionand thus a better suppression of the interference by the receiver.

The different methods (different frequency, temporal restriction) canalso be combined with one another and/or employed alternately, in orderto arrive at an improved interference suppression overall.

Furthermore it is also conceivable for there to be an averaging of theresults over longer or over a number of acquisition periods and/or alsobetween the different methods.

In another step the receiver, for example by means of an interferencesuppression controller, sets the parameter in such a way that a portionof the interference signal is reduced in the receive signal. It isconceivable for example for the parameter to be set by the interferencesuppression controller in an optimization method in such a way that anenergy of the interference signal is minimized in the receive signal.

As already explained previously for the receiver, the receiver in thiscase can also store the magnetic resonance signal and/or theinterference signal in this case so that the steps of processing and ofsetting the parameters can also take place spaced apart in time fromreceipt of the signals by the first receiving antenna and the secondreceiving antenna.

In an advantageous way, the inventive MRI scanner and the method foroperation by the second receiving antenna and the inventive receivermake it possible to reduce the portion of external interference signalsin the magnetic resonance signal and therefore to manage with simplerand lower-cost screening measures.

Further advantageous forms of embodiment are specified in the dependentclaims.

In a possible form of embodiment of the inventive MRI scanner, the MRIscanner is designed to receive magnetic resonance signals having aLarmor frequency in an industrial band. A resonant frequency of thenuclear spins used for the imaging in the MRI scanner in a staticmagnetic field BO of a field magnet of the MRI scanner is referred to inthis case as the Larmor frequency of the MRI scanner. Frequency bandsreleased for use by medical or technical devices are referred to here asthe industrial band, for which simplified regulations for emission andapproval are available. These are also referred to as ISM bands(Industrial, Scientific, Medical Band). An example of a frequency bandin which there may also be emissions with high powers, lies between 26.9and 27.3 MHz. Other such frequency bands lie between 6.7 MHz and 6.8MHz, 13.5 MHz and 13.6 MHz, 40.6 MHz and 40.7 MHz and also 433.0 MHz and434.8 MHz.

An MRI scanner is not only dependent on the lowest possible interferencebeing received, but on the strong excitation pulses also not interferingwith other devices. In the ISM band the legal tolerance limits aresignificantly higher, so that a legally-conformant restriction throughscreening of the emitted excitation pulses is more easily possible atthis frequency or even is not required. In synergistic connection withthe active screening in order to suppress receive-side interference byother devices in the ISM band, an MRI scanner can be realized in anadvantageous way entirely without a screening cabin.

In a possible form of embodiment the MRI scanner in this case the MRIscanner has a transmit path for transmitting the excitation pulses witha filter. The filter in this case is designed to suppress signalsoutside the ISM band. For example the filter can involve a bandpassfilter for the ISM band used, that damps out frequencies outside the ISMband by more than 12 dB, 24 DB, 40 dB or 60 dB relative to a signal withminimal attenuation within the ISM band. Depending on the implementationof the radio frequency generation of the MRI scanner, the filter can bearranged for example between final stage and a hybrid coupler, betweenhybrid coupler and transmit/receive switch or between transmit/receiveswitch and transmit antenna.

The filter makes it possible in an advantageous way to restrict radiofrequency power transmitted essentially to the ISM band and in this wayto adhere to the stricter limit values outside the ISM band. In this waythere can even be operation of the MRI scanner in the ISM band withoutan RF cabin.

Basically however it is also conceivable to use an ISM band with ascreening cabin without active interference suppression on the receiveside, in particular when the emissions by the excitation pulse arecritical for an approval. This also applies to the forms of embodimentdescribed below, which relate to a restriction or optimization of theexcitation pulse for the ISM band.

In a conceivable form of embodiment of the inventive MRI scanner, theMRI scanner has a transmit antenna for transmitting an excitation pulse,wherein the MRI scanner has non-linear components for tuning thetransmit antenna. These can be PIN diodes for example, but also otherdiodes or active components such as transistors or FETs. In this casethe non-linear components are arranged in an area of the MRI scannerscreened off from the patient tunnel for radio frequency and the filterfor the ISM band or ISM filter is arranged in the signal connectionbetween non-linear component and antenna, preferably in the screened-offarea.

In an advantageous way, by the screening-off of the non-linearcomponents from the patient tunnel and the environment, emissions fromfrequency components are avoided that are created by the non-linearityduring the excitation pulse by the components used for tuning and asharmonics no longer lie in the ISM. The filter prevents harmonics beingtransmitted via the signal connection between non-linear component andtransmit antenna. In this way the arrangement of the non-linearcomponents contributes to adherence to emission limit values and makesit possible or simplifies dispensing with the screening of the entireMRI scanner by a radio frequency cabin.

In a possible form of embodiment of the inventive MRI scanner, the MRIscanner has radio frequency unit with a preliminary interferencesuppressor. The preliminary interference suppressor is designed toprovide preliminary interference suppression of the excitation pulse forexcitation of the nuclear spins in such a way that the signal componentsof the excitation pulse on transmission, i.e. in particular afteramplification by a radio frequency power amplifier outside the ISM band,are reduced by comparison with an excitation pulse without preliminaryinterference suppression. It is conceivable for example for thepreliminary interference suppressor to create and mix-in signalcomponents, which after an amplification by the radio frequency unitcorrespond to the harmonics created by the non-linearity of the radiofrequency unit from the excitation pulse, but have a reversed leadingsign and in this way the harmonics reduce or extinguish each other. Thepreliminary interference suppressor in this case can be realized forexample in a digital signal generator or also corresponding signals canbe created from an input signal for a power amplifier by analogcomponents. The preliminary interference suppressor in this case canalso be adaptive, for example be controlled in its characteristics as afunction of a loading of the patient tunnel. Also conceivable is a partregulation by a fast feedback from a sensor in the transmit path, suchas e.g. a directional coupler.

In an advantageous way, the preliminary interference suppressor reducesharmonics outside the ISM band and in this way facilitates or makespossible adherence to the emission limit values even without a screeningcabin.

In a conceivable form of embodiment of the inventive MRI scanner, alimit frequency for a propagation of a radio wave in the patient tunnelis greater than a Larmor frequency of the MRI scanner. The limitfrequency is seen as the frequency at which a radio wave propagating inthe longitudinal (z direction) through the patient tunnel can still formin the patient tunnel as a hollow conductor. The limit frequency is alsoreferred to as the cut-off frequency for a waveguide, here for thepatient tunnel as a hollow conductor.

If the frequency of a radio signal lies below this, then in anadvantageous way there is an exponential drop in the field strength ofan interference signal coming from outside with the distance from theopening inside the patient tunnel, so that the interference signal issignificantly reduced in the examination region (Field of View, FoV).

In a possible form of embodiment of the inventive MRI scanner, thesecond receiving antenna is arranged at an opening of the patient tunnelor on the patient couch. For example the second receiving antenna can bearranged directly at an edge of an opening or below the surface on whichthe patient lies.

In particular, if the frequency of the interference signal lies belowthe limit frequency for a free wave, the patient and the patient tunnelform a coaxial conductor for the interference signal. A second receivingantenna in the vicinity of the patient and the opening can detect theinterference signal coupled into the patient tunnel through the patientin an advantageous way and in this way make possible an especiallyeffective suppression by the receiver.

In a possible form of embodiment, the MRI scanner has a waveguide thatsurrounds the MRI scanner, wherein the waveguide has a limit frequencyor cut-off frequency that is greater than the Larmor frequency of theMRI scanner. A waveguide in this case is seen as anyelectrically-conducting structure, which surrounds the MRI scanner atleast in four spatial directions around the outside, for example in theform of a tube or a prism, and through its conductivity at the Larmorfrequency of the MRI scanner essentially suppresses a propagation ofradio waves or electrical fields through the conducting structure. Inother words, on a side of the electrically-conducting structure of thewaveguide facing away from the MRI scanner a signal with Larmorfrequency is attenuated compared to a signal on the side facing towardsthe MRI scanner by more than 30 dB, 40 dB, 60 dB or more.

It is also conceivable for an inventive waveguide or even a classicalscreening cabin to surround the inventive MRI scanner in one form ofembodiment. A radio-frequency-proof door to the screening cabin or thewaveguide is replaced however in this case by an electrically conductingtunnel, which for its part represents a waveguide with a cut-offfrequency greater than the Larmor frequency.

Through the attenuation of the alternating fields in the tunnel,emission into the surroundings is reduced, so that an expensive RF-proofdoor that is difficult to use and susceptible to faults can be dispensedwith, in particular at the higher limit values for frequencies in ISMbands.

Here too use of a waveguide as door or screening is also basicallyconceivable without any active interference suppression in the receivepath.

In a conceivable form of embodiment of the inventive MRI scanner, thewaveguide has an electrically conductive connection to the patienttunnel. In this case it is conceivable for the waveguide together withthe patient tunnel to embody an end-to-end or contiguous waveguide. Itis also possible for the waveguide to be connected electrically at bothends of the patient tunnel to the patient tunnel or for two waveguidesat opposite ends of the patient tunnel to be connectedelectrically-conductively to said tunnel. In this case the waveguide canalso have a limit frequency or cut-off frequency that is different fromthe limit frequency of the patient tunnel but is likewise above theLarmor frequency of the MRI scanner.

In a conceivable form of embodiment of the inventive MRI scanner, thesecond receiving antenna has an essentially omnidirectional receivecharacteristic. An essentially omnidirectional receive characteristic isseen as a sensitivity distribution of the receiving antenna in allspatial directions, in which the difference between a maximumsensitivity and a minimum sensitivity as a function of the differentdirections is less than 6 dB, 12 dB, 18 dB or 24 dB. This preferablyalso applies to different polarizations of the interference signal.

The incident interference signal can come from different directions andwith different polarizations, so that an antenna with a directionalcharacteristic or preferred polarization cannot detect all interferencesignals. An antenna with an omnidirectional receive characteristic onthe other hand can detect all interference signals in an advantageousway.

In a possible form of embodiment of the inventive MRI scanner, the MRIscanner has a plurality of second antennas and the receiver is designedto suppress the interference signal in the magnetic resonance signal asa function of receive signals of the plurality of second receivingantennas. Preferably the plurality of second receiving antennas arearranged spaced apart from one another, for example at a distancegreater than a quarter of a wavelength or a half wavelength of a radiowave having the Larmor frequency.

In an advantageous way, a plurality of spatially distributed antennas isalso better suited to detecting one or more interference sources andthus to improving the interference suppression.

In a conceivable form of embodiment of the inventive MRI scanner, theplurality of receiving antennas is arranged in a symmetry arrangement inrelation to the patient tunnel. Conceivable for example is anarrangement at the edge of the opening of the patient tunnel at twoopposite points, on the corner points of a regular polygon or polyeder.

In an advantageous way, a symmetry relationship of the parameters canalso be produced by means of the symmetry of the antennas and in thisway the optimization method for reducing the interference signal can besimplified and/or speeded up.

In a possible form of embodiment of the inventive MRI scanner, the MRIscanner has an interference suppression transmitter and an interferencesuppression antenna. The interference suppression antenna is arranged ata distance from the patient tunnel. The interference suppressiontransmitter is designed in this case to generate a signal in a frequencyrange of an excitation pulse of the MRI scanner and to output it via theinterference suppression antenna in such a way that a field strength ofthe excitation pulse will be reduced by destructive interference in anarea surrounding the MRI scanner. Reduced is seen as reduction of thefield strength or attenuation of the signal of the excitation pulse inthe area by more than 6 dB, 12 dB, 24 dB, 40 dB or 60 dB.

In an advantageous way, the field strength of the electromagnetic wavesemitted by the excitation pulse in the environment of the MRI scannercan be reduced by the interference suppression transmitter and theinterference suppression antenna in such a way, in particular in synergywith the other proposed measures, that even without an RF cabin thelegal limit values can be adhered to.

In a conceivable form of embodiment of the inventive MRI scanner, theMRI scanner has a plurality of interference suppression antennas,wherein the interference suppression antennas are arranged at a distancefrom the patient tunnel and relative to one another. The distances arepreferably less than a wavelength of a free radio wave having the Larmorfrequency and/or greater than a tenth of the wavelength. For example theinterference suppression antennas can be arranged in a plane around theopening of the patient tunnel. The interference suppression transmitteris designed to output signals in a frequency range of an excitationpulse of the MRI scanner via the interference suppression antennas, sothat a field strength of the excitation pulse is reduced by destructiveinterference in a number of areas of an environment of the MRI scanner.

In an advantageous way, it is possible with a number of interferencesuppression antennas and activation signals of the interferencesuppression transmitter to reduce the electromagnetic fields in a numberof areas or to reduce the emissions in a number of directions or ideallyto bring them down to zero. The number of directions without emissionsin this case corresponds to zero points of the emission diagram of amultipole. A resulting electromagnetic wave from a signal of theexcitation pulse from a local transmit antenna and the interferencesuppression antennas is thus as a multipole field (e.g. quadrupolefield), that in an advantageous way drops significantly more quickly asthe distance from the source increases than a dipole field for exampleand in this way makes it possible to adhere to limit values foremissions even without an RF cabin.

In a possible form of embodiment of the inventive MRI scanner, theinterference suppression transmitter is designed to generate the signalsfor the interference suppression antenna or interference suppressionantennas by phase displacement and/or amplitude adaptation as a functionof one or more transmission interference suppression parameters.Preferably the respective signal or signals for the interferencesuppression antenna or interference suppression antennas are generatedfrom the excitation pulse by adjustable amplifiers or attenuators andphase displacement elements in the interference suppression transmitter.The amplitude relationships and phase displacements in this case canrepresent the transmit interference suppression parameters or be derivedfrom the transmit interference suppression parameter or transmitinterference suppression parameters, for example by analyticalfunctions, tables or iteration methods.

The excitation pulse in this case can be detected for example via asensor such as a directional coupler in the line between radio frequencypower amplifier and transmit antenna or a sensor antenna in the patienttunnel. It is also conceivable to generate the signals for theinterference suppression antennas directly from the digital data of theexcitation pulse by scaling and phase displacement with A/D convertersand amplifiers.

In an advantageous way, transmit interference suppression parameters, byreducing the number of variables, simplify a subsequently specifieddetermination of the setting of the interference suppression transmitterand make possible a more rapid adaptation to changed conditions such asother excitation pulses or being surrounded by patient or operatingpersonnel.

In a possible form of embodiment of the MRI scanner, the transmitinterference suppression parameters are set when the installation ismanufactured.

In a conceivable form of embodiment of the inventive MRI scanner, theMRI scanner has a calibration element in an environment of the MRIscanner and an interference suppression controller. The calibrationelement is preferably an antenna or sensor with which an electricaland/or magnetic field strength of an electrical and/or magneticalternating field with a frequency of the excitation pulse is detectedand can be forwarded to the interference suppression controller. Theinterference suppression controller is designed to detect a fieldstrength in a frequency range of an excitation pulse at the location ofthe calibration element by means of the calibration element. For examplethe interference suppression controller can have a calibration receiver.The interference suppression controller is furthermore designed,depending on the detected field strength, to set the transmitinterference suppression parameters in such a way that a field strengthof the excitation pulse is reduced in a predetermined environment of thecalibration element. For example the interference suppression controllercan vary or optimize an amplitude and/or phase of the interferencesuppression antenna or antennas in such a way that a reduction and/or alocal minimum of the field strength is obtained by destructiveinterference at the location of the calibration element.

In an advantageous way, by the adaptive setting of the transmitinterference suppression parameter or parameters, there can be areaction to an environment changed by persons or equipment in theenvironment of the MRI scanner, in order to adhere to the limit valuesfor emission even under changing conditions. If necessary thetransmission can be interrupted and/or a new determination of theparameters initiated if a limit value is exceeded.

In a possible form of embodiment of the inventive MRI scanner, theinterference suppression antenna has a radio frequency power amplifier.The radio frequency power amplifier is preferably designed in this caseto generate an electromagnetic alternating field sufficient forsuppression of stray fields of the excitation pulse via the interferencesuppression antenna from an activation signal with a low radio frequencypower, for example less than 10 mW, 50 mW or 100 mW.

The radio frequency power amplifier makes it possible, compared to apurely passive interference suppression antenna, to connect this by athin, flexible radio frequency cable to the interference suppressiontransmitter and in this way to simplify the installation. Inadvantageous interaction with the patient tunnel as waveguide and theplurality of interference suppression antennas, in this case only apower in the range of a few watts is required, so that the locallyarranged radio frequency power amplifier can be made small and light,which simplifies the installation.

In this case, the inventive transmission interference suppression bymeans of destructive interference can also be applied independently ofthe other features of the inventive MRI scanner. For example use of anactive interference suppression transmitter even without activereceive-side interference suppression is conceivable, in particular whenthe permitted electromagnetic radiation is the limiting factor.

Particular synergies are produced however in ISM bands by the higherpermitted limit values, which in conjunction with a receive-sideinterference suppression make possible operation even without closedscreening cabins.

In a conceivable form of embodiment of the inventive method, the step ofsetting the parameter has the step of an averaging over time with theformation of a temporal average value as a function of the interferencesignal. For example the amplitude and/or phase of the interferencesignal can be detected and averaged via a lowpass or by forming theaverage over a window, so that the parameter only follows slow changes.

In an advantageous way, the averaging over time leads to theinterference suppression not being falsified by short-term, possiblysporadic influences and to higher-frequency interference components notbeing artificially created by the interference suppression.

In a possible form of embodiment of the inventive method with an MRIscanner with calibration element, the setting step has the followingsub-steps:

In one step, the receiver measures a first transfer function between afirst receiving antenna and the calibration element. The measurement cantake place for example by the receiver or the interference suppressioncontroller instructing the interference suppression transmitter via asignal connection to transmit a signal with a predetermined amplitudeand/or predetermined phase in a frequency range of the magneticresonance signal and/or in an adjacent frequency range via thecalibration antenna. In this case a signal connection betweeninterference suppression transmitter and calibration element isnecessary. The receiver can then receive the signal via the firstreceiving antenna and define a first transfer function in this way. Thiscan also occur simultaneously for a number of first receiving antennas.Basically however it would also be conceivable to detect the transferfunction by transmitting via the first receiving antenna and receivingvia the calibration element.

In a further step, the receiver measures a second transfer functionbetween second receiving antenna and the calibration element. This cantake place in the same way as previously described. If the signal istransmitted via the calibration element, then in an advantageous wayboth transfer functions can be detected at the same time.

Preferably the signals transmitted for detection of the first and secondtransfer functions are encoded so that the receiver can establish theamplitude and phase relationship in a simple way. For example a pseudorandom code would be conceivable, which allows a fast and safeautocorrelation of the signals. The signal in this case can be modulatedin amplitude, frequency and/or phase. Also conceivable arespread-spectrum modulations, in which the signal for establishing thetransfer function can also remain below the noise limit of the MRsignals and a simultaneous transmission is possible during a detectionof an MR signal. In this way changes in the environment can be addressedpermanently. The permanent emission of the signal can also be achievedby use of a frequency range adjacent to the MR signals. Then however thedifferent propagation conditions by the different frequencies in thedetermination of the transfer functions must be taken into account.

In a further step the interference suppression parameter, or with anumber of first and second receiving antennas the interferencesuppression parameters are set as a function of the measured firsttransfer function or transfer functions and second transfer function ortransfer functions, so that a portion of an interference signal receivedby the second receiving antenna or antennas is reduced in a signalreceived by the receiver via the first receiving antenna. In this casethe interference signal received by the second receiving antenna orantenna preferably also continues to be taken into account. The transferfunctions in this case are applied to the form in which the signal of anindividual receiving antenna is taken into account. This can be achievedfor example by the interference suppression parameters for theinterference signal separated via an autocorrelation in the received MRsignal being set in such a way by a variation method or linearoptimization method by the interference suppression controller that theinterference signal portion is minimized. In this case the transferfunctions are included as predetermined attenuations and phasedisplacements between receiving antennas and receiver. For a calibrationelement, the transfer function in this case in precise terms is onlyvalid for an interference source at a specific location or direction.When sufficiently many and suitably positioned calibration elements areused to determine the transfer functions, transfer functions can also bedetermined independently of the respective location the calibrationelement.

In an advantageous way, the determination of the transfer functionsusing the calibration element makes it possible for the receiveinterference suppression to react to different conditions in theenvironment, such as for example position of people or equipment and thepropagation conditions changed thereby and to adapt the interferencesuppression.

In a possible form of embodiment of the inventive method, the settingstep occurs in a period of a sequence in which no magnetic resonancesignal is received, in particular also no signal for excitation of thespins is sent. Within a sequence for image acquisition there are periodsof time in which no excitation pulses are emitted and also there is noacquisition of a magnetic resonance signal for imaging. Preferably thisinvolves periods of the sequence in which the examination object or thepatient is not emitting any appreciable magnetic resonance signal, i.e.the level of the signal is at least 12 dB, 24 dB, 36 dB 48 dB or 60 dBbelow a maximum magnetic resonance signal. In this form of embodiment ofthe method the setting step takes place in such a period.

The interference suppression controller of the receiver iscorrespondingly designed to carry out the setting step in such a period.For example it can receive a trigger signal from the controller of theMRI scanner.

In an advantageous way, the setting of the parameters can be simplifiedwithout a magnetic resonance signal, for example by an energy of thesignal received by the first receiving antenna being minimized as afunction of the parameters. Even if the interference signal changes inits amplitude over time for example the parameters set continue toremain valid and effective with the same spatial arrangement.

In another possible form of embodiment of the inventive method, it isalso conceivable however for the setting of the parameter to take placepermanently, i.e. in short intervals of 1 ms, 10 ms or 100 ms orespecially in real time with a delay of less than 10, 100 or 500microseconds.

In an advantageous way, the permanent setting of the parameter in realtime or almost in real time allows there to be a reaction to newinterference signals occurring and their negative effect on the imagingto be minimized as much as possible.

In a further conceivable form of embodiment, the receiver has a memoryand stores the received magnetic resonance signal and the receivedinterference signal. The receiver in this case can for example in thesense of the invention also comprise an image evaluation processor. Itis however also conceivable in one form of embodiment for the receiverin the narrower sense, i.e. the devices used for editing the receivedradio frequency magnetic resonance signals, to include the memory. Thesetting of the parameters and the processing of the magnetic resonancesignal with the interference signal as a function of the parameter forreduction of the interference signal by the receiver to a receive signalcan then take place with a delay to the receipt. The delay can forexample comprise the duration of an echo sequence or also of an entireimage acquisition sequence, for example more than 10 ms, 100 ms, 0.5 s,10 s or even several minutes, hours or in principle also days.

In an advantageous way, the storage makes it possible to use resourcesof the MRI scanner already available as well or, on account of the factthat real time processing is not required, also to provide theinterference suppression at lower cost with less computing power. Theretroactive interference suppression also makes possible a comparison ofresults of different parameter settings and suppression methods and thusan optimization of the interference suppression.

In one conceivable form of embodiment of the inventive method, thereceiver or the interference suppression controller has anautocorrelation facility. The interference suppression controller candetermine a portion of the interference signal in the magnetic resonancesignal, for example an amplitude and a phase displacement, by means ofthe autocorrelation facility.

In another possible form of embodiment, the interference suppressioncontroller has an estimation device, which for example establishes theportion of the interference signal by an optimization method in whichthe interference portion is minimized in the magnetic resonance signalby variation of the parameter or the parameters, such as by Least MeanSquare Root (LSR) or similar methods.

With a number of parameters the interference suppression controlleroptimizes the plurality of the parameters in such a way that as large aportion of the interference signal as possible is reduced. This can beof advantage for example with a number of interference sources orreflections.

In an advantageous way, autocorrelation or estimation allow a flexibleadaptation to different interference sources.

In a possible form of embodiment of the inventive method, the step ofsetting the parameters has the following the following sub-steps:

In one sub-step, the received magnetic resonance signals are transformedinto an image space. In this case the usual methods used in MR imagingsuch as for example a Fourier transform are used, but also other methodssuch as compressed sensing are conceivable.

In another sub-step of the method, the interference signals areseparated in the image space from the magnetic resonance data. This cantake place for example by comparing two adjacent volumes or by two itemsof image data of the same volume acquired at different points in time.While the image data is the same or similar, image artifacts created byinterference signals are markedly different on account of the lack ofcorrelation.

It is also conceivable for the acquired image space or the associatedvolume to be greater than the examination object. Regions that do nothave any magnetic resonance signal, but merely interference signals, arethen detected in the image space. Through this segmentation theinterference signal can be separated and established.

In a further sub-step, the interference signals separated in the imagespace are transformed back into a raw data space or k-space, for exampleonce more with a Fourier transform.

In another sub-step, the parameters for the suppression of theinterference signals are determined from the transformed interferencesignals in the raw data space. For example the coordinates in thek-space specify information about phase and frequency of an interferencesignal, so that taking into account the arrangement of the firstreceiving antenna and second receiving antenna and also attenuationfactors and phase displacement of the signal paths, an attenuation andsignal delay can be determined, after the application of which to theinterference signal of the second receiving antenna in a sum signal withthe receive signal of the first receiving antenna, the interference inthe magnetic resonance signal is reduced.

It is also conceivable however for the interference signals to besuppressed directly in the image space, for example by hiding thecorresponding image data. This can be used in particular when the imagedata does not lie in the examination region or can be replaced by datawithout any interference already acquired from this region. It is alsoconceivable to identify the data with interference in the image space bya particular indicator, e.g. a color or brightness value, so that imageartifacts caused by interference cannot be taken for features of theexamination object.

In an advantageous way, the recognition of the interference signals inthe image space allows a separation or segmentation of magneticresonance data and interference. This enables the interference signalsto be acquired separately in a simple way and the characteristics to bebetter identified, which leads to a better and more effectivesuppression.

In a conceivable form of embodiment of the inventive method, thesub-steps of the transformation, of the separation, of the backtransformation and of the determination of the parameters on rows ofdata in the received magnetic resonance signals take place in the rawdata space.

Through the application of the inventive method to individual rows ofthe raw data space and their transforms in the image space, in anadvantageous way the parameters can be changed more quickly than forexample during acquisition of an entire slice and there can thus be afaster reaction to changes of the interference signals. A repetition ofthe acquisition for an entire slice can be avoided in this way.

In a possible form of embodiment of the inventive method, the receivermonitors the interference signal in one step for changes and adapts theparameters in a further step if there is a change. For example it ispossible, through a movement of the interference source or a reflectingobject in the environment of the MRI scanner, for the field distributionof the interference source to change in phase and amplitude. Thereceiver can then detect such changes of the interference signal andadapt the parameter or the parameters accordingly, so that for examplethe interference signal received by the second receiving antenna or thesecond receiving antennas is added to the MRI signal with adaptedamplification and/or phase displacement. In this case it is conceivablefor the receiver to take account of threshold values during themonitoring and for only a threshold value being exceeded to be seen as achange. Likewise it is conceivable for the receiver to undertake anaveraging over time in order to edit out short-term fluctuations. Theaveraging in this case can for example involve characteristics of theinterference signal such as amplitude, phase, frequency and/or frequencydistribution, in order not to react to short-term fluctuations and tointroduce as few additional noise components into the magnetic resonancesignal by the interference suppression as possible. It is alsoconceivable to average the parameters over a period of time or filterthem with a lowpass during the setting step.

In an advantageous way, a monitoring of the interference signal by thereceiver allows there to be a reaction to changes of the interferencesignal and in this way to insure effective noise suppression over alonger period of time of a variable interference signal. Thresholds andaveraging in this case allow the changes to be limited and variationscaused by instabilities or artifacts caused by compensation that is toogreat to be avoided.

In a possible form of embodiment of the inventive method, the receiverstores a first received magnetic resonance signal in a memory in onesub-step. In another sub-step the receiver stores a second receivedmagnetic resonance signal in a memory. In a further sub-step thereceiver compares the first received magnetic resonance signal and thesecond received magnetic resonance signal. If the receiver recognizes adeviation in this process that is to be attributed to externalinterference sources, the receiver carries out an interferencesuppression measure or signals a fault to the controller of the MRIscanner, so that this initiates the interference suppression measure. Inthis way sudden differences in amplitude in magnetic resonance signalsof slices lying close to one another or measurements of the same slice,for example from a calibration measurement, can possibly be interferencesignals.

In an advantageous way, interference signals can be recognized by thecomparison of the magnetic resonance signals of adjacent regions or thesame regions at different times. This is also conceivable in anapplication without second receiving antenna for receiving theinterference signal.

In a conceivable form of embodiment of the inventive method, theinterference suppression measure is the discarding of the first and/orsecond received magnetic resonance signal. It is also conceivable, inaddition or as an alternative, for an acquisition of the first and/orsecond magnetic resonance signal to be repeated. The interferencesuppression measure can also comprise a setting of the parameter.

Through the proposed interference suppression measures, image artifactsin the magnetic resonance recordings after recognition of aninterference signal can be avoided.

In a possible form of embodiment of the inventive method for operationof an MRI scanner, the MRI scanner has a patient tunnel, a firstreceiving antenna for receiving a magnetic resonance signal from apatient in the patient tunnel, a second receiving antenna for receivinga signal with the Larmor frequency of the magnetic resonance signal anda receiver, wherein the second receiving antenna is arranged outside thepatient tunnel or in the vicinity of an opening of the patient tunnel.The method has the step of receipt of an interference signal by thereceiver via the second receiving antenna. Through the arrangement ofthe second receiving antenna, said antenna is more sensitive to signalsoutside the patient tunnel, which merely because of their spatial origincannot be magnetic resonance signals. Basically the signal of the secondreceiving antenna can basically also have small portions of a magneticresonance signal, but which because of their small portion can be notconsidered initially for the interference suppression or can be furtherreduced or avoided with means and methods specified in dependent claims.

In another step, the receiver receives a magnetic resonance signal viathe first receiving antenna. In this case the signal that is primarilyused for image acquisition is referred to as the magnetic resonancesignal. For example the first receiving antennas can involve local coilson the body of the patient. The magnetic resonance signal can containportions of the interference signal, which are to be further reducedwith the inventive facility or method as described below.

In a further step, the received magnetic resonance signal of the firstreceiving antenna is discarded depending on the interference signalreceived by the second antenna. For example the interference signalreceived by the second receiving antenna can lie above a thresholdlevel, so that despite the propagation attenuation between location ofthe second receiving antenna and location of the first receivingantenna, a fault is to be expected from the image obtained from themagnetic resonance signal. The receiver or the control of the MRIscanner can then discard the signal of the first receiving antenna. Itis then conceivable for the MRI scanner subsequently to repeat theacquisition of the discarded signal.

In an advantageous way, an interference signal can already be recognizedon the basis of the signal amplitudes by the different arrangement offirst and second receiving antenna.

In a conceivable form of embodiment of the inventive method foroperation of an MRI scanner having a Larmor frequency in an ISM band,the method has the step of determining an excitation pulse forexcitation of nuclear spins in an examination object. The determinationin this case takes place as a function of predetermined frequency limitsof the ISM band. Preferably in this case the excitation pulse isdetermined in such a way that it only has spectral portions outside theISM band below predetermined threshold values. The restriction to theISM band can be achieved for example by the measures described forclaims given below.

In another step of the method, the MRI scanner emits the excitationpulse.

In a further step of the method the MRI scanner receives a magneticresonance signal and in another step establishes a mapping of adistribution of nuclear spins in the examination object. The mapping cansubsequently be output on a display.

Within the ISM band, markedly higher limit values for radio wavesemitted into the environment are usually allowed. Excitation pulses,depending on slice density and duration can be so wideband that theyhave portions outside the ISM band, even if the Larmor frequency lieswithin the ISM band. If this is avoided by the measures explained below,then additional measures such as an RF cabin can dispensed with, withoutexceeding the limit values.

In a possible form of embodiment of the inventive method the step ofdetermining an excitation pulse has the sub-steps of determining anexcitation pulse to excite the nuclear spins in a slice of theexamination object as a function of a relative position of the slice fora magnet unit, of a predetermined gradient strength, of a thickness of aslice and of the type of measurement. This can take place for example byusing parameterized libraries on excitation pulses.

In a further test step it is established by the MRI scanner whether theexcitation pulse lies within the predetermined frequency limits. Thiscan take place for example by spectral analysis by means of FFT.

If the excitation pulse does not lie within the predetermined frequencylimits, the establishing steps are repeated. In these a pulse parameteris varied, which on establishing the excitation pulse has an effect on aspectral frequency distribution of the excitation pulse.

If the excitation pulse lies within the predetermined frequency limits,it is emitted in a further step by the MRI scanner.

In a conceivable form of embodiment of the inventive method, the pulseparameter with an effect on the establishing of the excitation pulse isone of the parameters duration of the excitation pulse, thickness of theslice, relative position of the slice or strength of the gradients. Inother words, to obtain a different excitation pulse that lies within theISM frequency band having the Larmor frequency of the MRI scanner, theduration of the excitation pulse, the thickness of the slice to beexcited or also the position of the slice or the strength of thegradients can be changed for example. A simultaneous variation of anumber of parameters is also conceivable.

By variation of one or more pulse parameters, an excitation pulse can beestablished in an advantageous way that does not exceed the limits ofthe frequency band and in this way makes operation within the permittedframework possible.

In a possible form of embodiment of the inventive method, the step ofemission has the sub-step of altering the position of the examinationobject relative to the magnet unit before the step of emitting thepulse.

Through the gradient fields, it can occur at the edges of the FoV thatthe resonant frequency of the nuclear spins deviates more strongly fromthe average Larmor frequency of the MRI scanner established by the BOfield. By repositioning the slice to be acquired more towards theisocenter of the BO field, with the same bandwidth of the excitationpulse, a complete excitation of a slice can be obtained, withoutdeparting from the band limits of the ISM band.

In a possible form of embodiment of the inventive MRI scanner, the MRIscanner has a control unit for controlling the image acquisition, whichin particular can influence parameters of the image acquisition such astime of the excitation pulse and/or time of the acquisition of themagnetic resonance signal or also frequency of the excitation pulseand/or frequency range of the receiver on receipt of the magneticresonance signal. Furthermore the MRI scanner has an interface connectedto the control unit for signaling. The interface, as explained below,can be an interface for exchange of data with other magnetic resonancesystems or also a radio frequency interface. The control unit isdesigned to synchronize an image acquisition as a function of a signalreceived by another MRI scanner via the interface. Synchronization hereis seen as any activity that reduces mutual interference. This cancomprise time coordination, but also for example a change offrequencies.

In an inventive MRI scanner, the control unit can also be designed totransmit a signal with information about an impending image acquisitionto another MRI scanner. This feature, in a similar way to a plug andsocket connection, supplements the aforementioned MRI scanner, which isdesigned to receive a signal from another MRI scanner. The informationpreferably relates to a time of emission and/or frequency of anexcitation pulse. Preferably an inventive MRI scanner, as explainedbelow, is designed both for transmitting and also for receiving asignal. The MRI scanner will also be referred to below as the first MRIscanner.

A signaling connection can also be established between the inventivefirst MRI scanner and a second inventive MRI scanner. The first MRIscanner and the second MRI scanner each have an interface and a controlunit for this purpose. The first MRI scanner and the second MRI scannerare connected for signaling via the interfaces. The signal connectionmakes it possible at least to transmit a signal from the first MRIscanner to the second MRI scanner, but a bidirectional exchange ofinformation is also conceivable. Point-to-point connections on anelectrical, optical or wireless path are conceivable. Networks such asLAN, WAN, specifically TCP/IP, are also possible as a signal connection.The control unit of the first MRI scanner is designed to transmitinformation for an impending image acquisition process via the interfaceto the second MRI scanner. A time of a planned emission of an excitationpulse in absolute time or relative to the signal, or also to thefrequency of a frequency range of the excitation pulse is conceivable.The control unit of the second MRI scanner is designed in this case toreceive the information via the interface and to carry out an imageacquisition as a function of the received information. For example thecontrol of the second MRI scanner can be designed to shift theacquisition of a magnetic resonance signal, i.e. a sequence or a partthereof, in time so that it does not disturb the excitation pulse of thefirst MRI scanner. The second MRI scanner is also referred to below asthe other MRI scanner.

The inventive method for operation of an MRI scanner can also beexpanded for operation of a first MRI scanner with a control unit forcontrolling the image acquisition and an interface connected to thecontrol unit for signaling. The method then has the step of receipt of asignal by the control unit via the interface from a second MRI scanner.This can involve an explicit exchange of information, in which the firstMRI scanner receives information or a message from the second MRIscanner via a data interface, which features parameters such as timeand/or frequency of an intended image acquisition. It is alsoconceivable however for the first MRI scanner to monitor the environmentfor example via a receiver for magnetic resonance signals.

In another step the control unit of the first MRI scanner sets aparameter of the image acquisition as a function of the received signal.It is for example conceivable for a sequence to be shifted in time.

In a further step, an image acquisition according to the parameter setis carried out by the first MRI scanner. For example the sequence can bebegun at a changed point in time, so that the excitation pulses of bothMRI scanners occur at the same time or the excitation pulse of the firstMRI scanner occurs at a point in time at which the second MRI scanner isnot receiving any image-relevant magnetic resonance signals.

In an advantageous way, the inventive MRI scanner, in conjunction with asecond MRI scanner and the method for operation, enable mutualinterference by two MRI scanners during simultaneous operation to bereduced.

In a possible form of embodiment of the inventive MRI scanner, theinterface is designed for the exchange of data. In other words, thefirst MRI scanner is designed both to transmit data via the interfaceboth to a second MRI scanner and also to receive data via the interfacedata from a second MRI scanner. In this case the control unit isdesigned, by means of exchange of information via the interface, tosynchronize an image acquisition with a second MRI scanner. In otherwords the control units agree between themselves by messages how theimage acquisition takes place, so that mutual disruptions are reduced.

In a conceivable form of embodiment of the inventive MRI scanner, thesignal has information about a time and/or frequency of a transmitprocess. The time in this case can be specified in absolute terms forexample or relative to the time at which the message was sent. A midfrequency and/or a bandwidth, a frequency range or also a channelspecification that encodes a frequency range can be given as aspecification.

In a possible form of embodiment of the inventive method for operationof a first MRI scanner and a second MRI scanner, the method furthermorehas the step of establishing information about an impending imageacquisition of the second MRI scanner by a control unit of the secondMRI scanner and in a further step of transmitting a signal with theinformation to the first MRI scanner.

For example the control unit can receive via the interface a messagefrom a second MRI scanner that said scanner will transmit an excitationpulse with a mid frequency equal to the Larmor frequency+100 kHz with abandwidth of 200 kHz in 2 seconds. The first MRI scanner can then forexample interrupt a sequence before its own excitation pulse andcontinue after the second MRI scanner has ended its sequence, ortransmit the next excitation pulse simultaneously with the excitationpulse of the second MRI scanner. In an advantageous way mutualdisruption by an excitation pulse of the second MRI scanner during areceive phase can be avoided in this way.

In a possible form of embodiment of the inventive MRI scanner, thesignal has information about a time and/or frequency of a receiveprocess. The information can specify a beginning or a duration andfrequency for example, as to how there will be receiving from the secondMRI scanner beginning in a second for 2 seconds at a mid frequency equalto the Larmor frequency minus 300 kHz with a bandwidth of 200 kHz. Thefirst MRI scanner can then for example interrupt a sequence in such away that it does not transmit an excitation pulse in this time in thesaid frequency band.

In an advantageous way, even on receipt of a message about a plannedreceipt in other units, an emission of the first MRI scanner can beshifted so that a disruption is reduced.

In a possible form of embodiment it is conceivable for the control unitof the first MRI scanner to be designed to change the frequency of animage acquisition process as a function of the received information. Forexample it is conceivable for an image acquisition to comprise differentslices, wherein these are differentiated by a gradient magnetic field inthe z-axis and thus in the effective Larmor frequency of the magneticresonance signal. In this way a simultaneous operation with reducedinteraction is possible, provided the order of the slices in the imageacquisition is arranged so that there is never an acquisition in thesame frequency range in both systems.

In an advantageous way, the differentiation via the frequency makespossible a simultaneous acquisition of the magnetic resonance signals inadjacent MRI scanners and thus a better use of the examination time.

In a conceivable form of embodiment of the inventive MRI scanner, thefirst MRI scanner has a receiver as the interface. A receiver here isseen in particular as a receiver for magnetic resonance signalsincluding an antenna such as a local coil or body coil. In this case thefirst MRI scanner is designed to acquire an excitation pulse of a secondMRI scanner outside an image acquisition and to carry out the imageacquisition as a function of the acquired excitation pulse. For exampleit is conceivable for the first MRI scanner then initially to acquire orwait for magnetic resonance signals of a slice with another effectiveLarmor frequency, until a maximum duration for an acquisition ofmagnetic resonance signals has elapsed in the second MRI scanneremitting the excitation pulse.

In this way, the synchronization can be undertaken in an advantageousway even without a data connection or changes in the second MRI scanner.

In a possible form of embodiment of the inventive method for operationof a first MRI scanner and a second MRI scanner, the method furthermorehas the step of establishing information about an impending imageacquisition of the second MRI scanner by a control unit of the secondMRI scanner and in a further step of transmitting a signal with theinformation to the first MRI scanner.

Basically it is also conceivable for the different measures described tobe combined with one another. A protocol for an exchange of informationin two MRI scanners can also be provided, through which the imageacquisitions from both systems are interleaved with one another in anoptimized way, so that the duration of the acquisition only changesslightly without any mutual disruptions occurring. In the simplest caseall excitation pulses could be undertaken at the same time for example,provided an image acquisition is taking place in both MRI scanners inthe same period of time.

The characteristics, features and advantages described above as well asthe manner in which these are achieved will become clearer and easier tounderstand in conjunction with the description given below of theexemplary embodiments, which will be explained in greater detail inconjunction with the drawings, in which:

FIG. 1 shows a schematic diagram of an MRI scanner with the inventivefacility;

FIG. 2 shows a schematic diagram of the receiver and the first receivingantenna and the second receiving antennas;

FIG. 3 shows a schematic diagram of a flowchart of a form of embodimentof the inventive method;

FIG. 4 shows a schematic diagram of a radio frequency unit of aninventive MRI scanner;

FIG. 5 shows a schematic diagram of an inventive MRI scanner surroundedby a waveguide;

FIG. 6 shows a schematic diagram of an inventive MRI scanner with aninterference suppression transmitter;

FIG. 7 shows a schematic diagram of a flowchart of a part aspect of theinventive method;

FIG. 8 shows a schematic diagram of a flowchart of a part aspect of theinventive method;

FIG. 9 shows a schematic diagram of a flowchart of a part aspect of theinventive method;

FIG. 10 shows a schematic diagram of a flowchart of a part aspect of theinventive method;

FIG. 11 shows a schematic diagram of an inventive MRI scanner, in anetwork with further MRI scanners;

FIG. 1 shows a schematic diagram of a form of embodiment of an MRIscanner 1 with an inventive local coil 50.

The magnet unit 10 has a field magnet 11, which creates a staticmagnetic field BO for alignment of nuclear spins of samples or of thepatient 100 in a recording region. The recording region is characterizedby an extremely homogeneous static magnetic field BO, wherein thehomogeneity relates in particular to the magnetic field strength or theamount. The recording region is almost spherical and is arranged in apatient tunnel 16, which extends in a longitudinal direction 2 throughthe magnet unit 10. A patient couch 30 is able to be moved in thepatient tunnel 16 by the drive unit 36. Usually the field magnet 11involves a superconducting magnet, which can provide magnetic fieldswith a magnetic flux density of up to 3T, with the latest devices evenbeyond this. For lower field strengths however permanent magnets orelectromagnets with normally conducting coils can be used.

Furthermore the magnet unit 10 has gradient coils 12, which are designedfor spatial differentiation of the acquired imaging regions in theexamination volume by superimposing on the magnetic field BO variablemagnetic fields in three spatial directions. The gradient coils 12 areusually coils made of normally conductive wires, which can create fieldsorthogonal to one another in the examination volume.

The magnet unit 10 likewise has a body coil 14, which is designed toirradiate a radio frequency signal supplied via a signal line into theexamination volume and to receive resonant signals emitted from thepatient 100 and output them via a signal line.

A control unit 20 supplies the magnet unit 10 with the various signalsfor the gradient coils 12 and the body coil 14 and evaluates thereceived signals.

In this way the control unit 20 has a gradient controller 21, which isdesigned to supply the gradient coils 12 with variable currents viasupply lines, which when their timing is coordinated, provide thedesired gradient fields in the examination volume.

Furthermore the control unit 20 has a radio frequency unit 22, which isdesigned to generate a radio frequency pulse with a predeterminedtemporal course, amplitude and spectral power distribution to excite amagnetic resonance of the nuclear spins in the patient 100. In this casepulse powers in the kilowatt range can be achieved. The excitationpulses can be irradiated into the patient 100 via the body coil 14 oralso via a local transmit antenna.

A controller 23 communicates via a signal bus 25 with the gradientcontrol 21 and the radio frequency unit 22.

A local coil 50 is arranged on the patient 100 as a first receive coil,which is connected to the radio frequency unit 22 and its receiver via aconnecting line 33. It is also conceivable however for the body coil 14to be a first receiving antenna in the sense of the invention.

Arranged at an edge of the opening of the patient tunnel 16 are foursecond receiving antennas 60, which are arranged at the corners of asquare, which is encompassed by the circular opening, so that thecorners lie on the edge of the opening. The four second receivingantennas 60 are connected to the receiver 70 of the radio frequency unit22 for signaling. As a result of the plurality of second receivingantennas 60 it is conceivable in this case for these not to all have anomnidirectional receive characteristic, but to be dipoles for exampleand to supplement each other to form an omnidirectional receivecharacteristic by their different alignment. It would also beconceivable however for example to provide a crossed-dipole as a singlesecond antenna with omnidirectional receive characteristic.

It is also possible, as an alternative or in addition, for a secondreceiving antenna 60 to be arranged in the patient couch 30.

The patient tunnel in this case preferably has a radius R for which thefollowing applies:

R<(Lambda_(L)*1.841)/(2*Pi)

Lambda_(L) in this case specifies the wavelength of a radio wave in airat the Larmor frequency of the MRI scanner 1. If the radius R is lessthan the right-hand term, then the radio wave propagates exponentiallyattenuated in the patient tunnel 16 and the interference signal isheavily attenuated in the middle in the examination region FoV.Lambda_(L) is also referred to as the limit wavelength of a round hollowconductor, the associated frequency as the limit frequency.

Only the patient 100 acts through their finite conductivity as the coreof the coaxial cable, the sheath of which is the wall of the patienttunnel 16, and passes on an electromagnetic signal coupled in at thelegs or the top of the head into the examination region. In anadvantageous way the second receiving antenna 70 or the second receivingantennas 70 arranged in the vicinity of the opening or in the patientcouch 30 in this case receive the interference signal passed on from thepatient 100 into the FoV and thereby make the compensation in thereceiver 70 especially effective.

FIG. 2 shows a schematic diagram of the functional units of a possibleform of embodiment of a receiver 70.

The summation device 71 weights the signals entering from the firstreceiving antenna or local coil 50 and from the second receivingantennas 60 with parameters, which can also be complex, in order tospecify a phase displacement. In an analog receiver 70 this can takeplace by an adjustable amplifier in conjunction with an adjustable phaseshifter. The real part of the parameter then corresponds to theamplification factor and the imaginary part of the phase displacement.Subsequently, in a preferred form of embodiment the weighted signals aresummed, but other non-linear signal operations for combination of theindividual signals are also conceivable.

An interference suppression controller 72 receives the combined signaland also the individual signals of the first receiving antenna and thesecond receiving antennas 60. In order to determine a portion of theinterference signal in the combined signal, it is conceivable forexample for the interference suppression controller 72 to undertake anautocorrelation of the signals. However it is also conceivable for theenergy of the combined signals to be determined. In a conceivable formof embodiment the interference suppression controller 72 determines theportion of the interference signal in sections of sequences of the MRIscanner in which no magnetic resonance signal for imaging is expected,so that the combined signal only has the interference signal. This canbe the case for example in dephased sections of an echo sequence, sincethe amplitudes of individual nuclear spins cancel each other out becauseof their different phase and do not generate any signal overall.

The interference suppression controller 72 then optimizes the parametersin the summation device according to a Least Square Root (LSR) methodfor example, so that the portion or the energy of the interferencesignal in the combined signal is minimized.

Basically the receiver 70 can be designed both in analog signalprocessing technology, so that for example gain control and phasedisplacement are controlled by a parameter and then converted usinganalog means, or also as a digital receiver, which either alreadyreceives digitized signals from the first receiving antenna and/or thesecond receiving antenna 60 or already digitizes said signals at thesignal input by means of an A/D converter.

For imaging, the receiver 70 forwards the combined signal, in which theinterference signal is very largely suppressed, to the controller 23 ofthe MRI scanner.

In this case it is also possible for the bandwidth of the interferencesignal to be wider than the magnetic resonance signal. The portionsoutside the frequency range of the magnetic resonance signal usuallycorrelate in this case with the portions within the frequency range ofthe magnetic resonance signal. Therefore it can also be sufficient inaccordance with the invention for the second receiving antenna 60 toreceive the wideband interference signal only in part, for example in afrequency range that is not the same as or is outside the frequencyrange of the magnetic resonance signal. In addition or as an alternativethe receiver 70 can also be designed only to accept frequencies in thisfrequency range from the second receiving antenna. The receiver 70 isdesigned in this case, depending on this part signal, to suppress theinterference signal in a magnetic resonance signal received by the firstreceiving antenna. Because of the correlation for example the amplitudeof the frequency components of the interference signal in the frequencyrange of the magnetic resonance signal can be linked with an amplitudeoutside the frequency range. In this way it is also conceivable for thesecond receiving antenna 60 in conjunction with the receiver 70 only tomonitor frequency components outside the frequency range of the magneticresonance signal for interference signals during an MRI scan and tosuppress the interference signal in the signals of the first receivingantenna as a function thereof. It would be possible in this case forexample for the receiver 70 to establish a relationship betweeninterference signal from the first receiving antenna and interferencesignal portion from the second receiving antenna 60, also referred to asthe transfer function, for an interference signal in the frequency rangeof the magnetic resonance signals between acquisitions of magneticresonance signals and during the acquisition of magnetic resonancesignal for example to adapt the amplitude or scaling of receivedportions of the interference signal on the basis of the second receivingantenna outside the frequency range of the magnetic resonance signal.

In another conceivable form of embodiment of the inventive MRI scanner(1), the interference signal is not suppressed in the receive signal inreal time, i.e. not immediately on receipt of the interference signaland/or of the magnetic resonance signal, but the magnetic resonancesignal and the interference signal are stored by the receiver 70, whichin this case can also comprise parts of the controller 23 of the MRIscanner 1 or of an image evaluation unit, in a memory. The steps set outbelow for the method then no longer take place in real time or almost inreal time, but can be carried out on the stored data with a delay, forexample in advance of an image evaluation.

In this case for example a combination of receipt with the secondantenna outside the frequency band of the MR signal with a real timeinterference suppression or a suppression at a separate point in time,for example during image evaluation, is conceivable.

The interference suppression in the receiver shown in FIG. 2 can also becarried out with a single second antenna 60. Conversely it is possiblefor the receiver 70 to have a number of channels or for a number ofreceivers 70 to be provided in the MRI scanner 1, in order to suppressinterference in the magnetic resonance signals of a number of localcoils 50. In this case it is conceivable for the signals of the secondreceiving antennas 60 in this case to be used by a number of receivers70 or channels of the receiver 70 for interference suppression.

In this case it is both conceivable for the dependence between theinterference signal received by the second antenna and the interferencesuppression to be linear and also non-linear. Linear dependencies inthis case can be a phase displacement about a value determined or alinear scaling with a value determined from the signal of the secondantenna. It is also conceivable however for the transfer function forthe interference signal to have non-linearities on the path from thefirst receiving antenna and/or second receiving antenna, for examplethrough mixers or non-linear amplifiers, so that non-linear operationsfor interference suppression in the receiver 70 must also be applied tothe interference signal received by the second receiving antenna 60.

FIG. 3 shows a schematic flowchart of an inventive method.

In a step S10, the receiver 70 receives an interference signal via thesecond receiving antenna 60 or via the plurality of second antennas 60.The interference signal is transmitted via a signal connection to thereceiver 70. In this case it is also conceivable for the interferencesignal to be digitized first of all by an A/D converter, before it istransmitted to the receiver 70, embodied in this case as a digitalreceiver 70.

In a step S20, the receiver 70 receives a magnetic resonance signal viathe first receiving antenna, for example the local coil 50. With anumber of local coils 50, correspondingly more receivers 70 can beprovided or also one receiver 70 with a number of channels, each with asummation device 71. The interference suppression controller 72 in thiscase can be provided separately in each case or also jointly, which canspeed up the subsequent setting or similar parameters for differentchannels.

In a step S30, the receiver 70 processes the magnetic resonance signalas a function of the interference signal or the interference signalswith a number of second receiving antennas 60 into one receive signal.For example the interference signal or signals of the second receivingantenna or second receiving antennas 60 and the magnetic resonancesignal of the first receiving antenna are weighted with differentparameters and delayed and subsequently combined. This can be thecreation of a linear combination for example. The created receive signalor sum signal depends in this case on one or more parameters.

In another step S40, the parameter or the parameters are set by thereceiver 70, in particular by the interference suppression controller 72in such a way that a portion of the interference signal in the receivesignal is reduced. If for example the interference signal received bythe second receiving antenna 60 is scaled by the parameters set so thatit has the same amplitude as the interference signal portion receivedvia the first receiving antenna and if it is provided with a phasedisplacement relative thereto of 180 degrees, then the interferencesignal in the created signal is exactly canceled out. The parameter orthe parameters can be established in this case via optimization methodssuch as for example Least Square Root (LSR) or Wiener filter.

The step S40 in this case can also have the sub-step S41, to form atemporal average value and to set the parameter for interferencesuppression as a function of this average value. For example anamplitude or phase of the interference signal can be averaged in orderto compensate for statistical fluctuations and introduce lessinterference into the magnetic resonance signal through the interferencesuppression.

In this case it is also conceivable for the steps S10 to S30 to becarried out in each case on received magnetic resonance signals andreceived interference signals in real time, in particular with analogreceivers 70. However it is also conceivable for the steps S10 to S30 tobe carried out in each case on stored interference signals and magneticresonance signals, which are digitized for example for an individualsequence or individual sections thereof.

In a possible form of embodiment of the inventive method the step S40 iscarried out with interference signals from a period of a sequence inwhich no magnetic resonance signal for imaging is received. For examplethe parameters can be determined with interference signals of the secondreceiving antenna 60 and a signal of the first receiving antenna in atime section of the interference suppression controller 72 in which thenuclear spins are dephased and create no magnetic resonance signal.However it is also conceivable in this time section without magneticresonance signal for the interference signal and the signal of the firstreceiving antenna to just be acquired digitally and evaluated later.

In this case it is conceivable in a step S50 for the interferencesuppression controller 72 to monitor the interference signal forchanges, for example different amplitude, frequency or phase. Ondetection of such a change or if the change exceeds a predeterminedthreshold value, the interference suppression controller 72 can modifythe steps S10 of receipt of interference signal and S20 receipt of themagnetic resonance signal as well as the setting of a parameter forinterference suppression as a function of the received interferencesignal in a step S51, in order to adapt the interference suppression tothe changed interference signal.

Basically it is also conceivable in a form of embodiment of theinventive method for the received interference signal and/or magneticresonance signal to be stored by the receiver 70 in a step S25. In thiscase the receiver 70 can also comprise parts of the controller 23 of theMRI scanner 1 or an external image evaluation processor. The steps S20to S40 are carried out retroactively in this case, for example at theend of an echo sequence, an excitation sequence, a signal acquisitionfor a slice of the examination object or also after acquisition of alldata.

The decoupling of the signal acquisition from interference signal andmagnetic resonance signal from the interference suppression thus make itpossible in an advantageous way to employ lower-cost components withlower computing power or also to use existing resources, e.g. from theimage evaluation, twice. It is also then conceivable to compare andselect different parameter settings or optimize them retrospectively.The application can also be limited to periods of time with disruptions.

FIG. 4 shows a schematic diagram of a radio frequency unit of aninventive MRI scanner. The diagram in this case does not show alldetails of the radio frequency unit, but only those relevant for aninventive interference suppression on the transmit path.

The transmit path of the radio frequency unit 22 in this case has apulse generator 220, a preliminary interference suppressor 221, a poweramplifier 222 and an ISM filter 223.

The pulse generator 220 can have an oscillator, a modulator and a mixerfor example, with which a pulse is generated in the baseband is thenconverted to the Larmor frequency.

The preliminary interference suppressor 221 is designed for preliminaryinterference suppression of an excitation pulse for excitation of thenuclear spins in such a way that portions of the signal of theexcitation pulse outside the ISM band are reduced compared to anexcitation pulse without preliminary interference suppression. It isconceivable for example for preliminary interference suppressor 221 tocreate and mix-in signal portions that, after an amplification by thepower amplifier, correspond to harmonics created by the non-linearity ofthe power amplifier from the excitation pulse, but have a reversed signand in this way reduce or extinguish the harmonics. The same isconceivable for intermodulations of signal portions by thenon-linearity. The preliminary interference suppressor 221 in this casecan be realized for example in a digital signal creation or alsocorresponding signals are created from an input signal for a poweramplifier by analog components. It is also conceivable for thepreliminary interference suppressor 221 to be integrated in a digitalpulse generator 220 for example.

The output signal of the preliminary interference suppressor 221 isamplified in the power amplifier 222. A power amplifier 22 with a linearcharacteristic curve is conceivable here. It is also possible howeverfor the preliminary interference suppressor 222 to change the inputsignal of the power amplifier 222 in just such a way that, afteramplification by the power amplifier 222, a signal is generated withoutunwanted harmonics. In other words the characteristic curve of thepreliminary interference suppressor 221 multiplied by the characteristiccurve of the power amplifier 222 ideally gives a linear characteristiccurve, so that the system consisting of preliminary interferencesuppressor 221 and power amplifier 222 amplifies a pulse of the pulsegenerator without unwanted harmonics.

The preliminary interference suppressor 221, in one form of embodiment,as shown in FIG. 4 by the dashed line from the output of the ISM filter223, can also be adaptive in the sense that, by monitoring the outputsignal of the power amplifier 222, it adapts the preliminaryinterference suppression, so that the overall system, consisting ofpreliminary interference suppressor and power amplifier, has a linearcharacteristic.

Subsequent to the power amplifier 222 the signal is preferably stillfiltered by an ISM filter 223. The filter in this case preferablysuppresses frequency components outside the ISM band that the MRIscanner 1 needs for image acquisition. For example the filter caninvolve a bandpass filter for the ISM band used that attenuatesfrequencies outside the ISM band by more than 12 dB, 24 DB, 40 dB or 60dB relative to a signal with minimal attenuation within the ISM band. Alowpass is also possible. Depending on the radio frequency generation ofthe MRI scanner the filter can be arranged for example between poweramplifier 222 and a hybrid coupler not shown, between hybrid coupler anda transmit/receive switch not shown or between transmit/receive switchand transmit antenna.

In a form of embodiment the MRI scanner also has tuning elements for thetransmit antenna. These can be PIN diodes for example, but also otherdiodes or active components such as transistors or FETs. These tuningelements are provided in order to tune the transmit antenna in the caseof receipt and to avoid interactions with the receiving antennas. Thetuning elements usually have non-linear characteristic curves cantherefore generate harmonics during transmission. In a preferred form ofembodiment the ISM filter 223 is therefore arranged between the tuningelements and the transmit antenna.

The non-linear components are also preferably arranged in an area of theMRI scanner screened off from the patient tunnel for radio frequency.

Thus the arrangement of the non-linear components contributes toadherence to irradiation limit values and makes possible or simplifiesdispensing with screening of the entire MRI scanner with a radiofrequency cabin.

Shown schematically in FIG. 5 is a form of embodiment of an inventiveMRI scanner 1, surrounded by a waveguide. The waveguide 260 can beprovided in this case by any electrically conductive surface surroundingthe outer circumference of the MRI scanner 1 in at least 4 spatialdirections. An electrically conductive surface is seen here inparticular as a metallic or metalized surface or mesh, which attenuatesan electromagnetic wave with the Larmor frequency at a crossing by 60dB, 80 dB, 100 dB or more. The conductivity of the surface in this casecan also be anisotropic through geometrical subdivision such as slots,provided the conductivity in parallel to the electrical field vector ofthe alternating field is sufficient to achieve the attenuation.

Preferably in this case the surface forms the waveguide 260 as a tunnelaround the MRI scanner e.g. in the form of a cylinder, cube, prism witha width that does not allow any formation of a free wave with Larmorfrequency. With a cube this is the case for example if the longerdimension of the cross section is less than a half wavelength of anelectromagnetic wave with the Larmor frequency. In other words, thecut-off frequency or limit frequency of the waveguide 260 is greaterthan the Larmor frequency. The electromagnetic field thereby fallsexponentially with the distance from the source, so that alternatingfields leaking out of the patient tunnel 16 fall rapidly. It isconceivable in this case for the waveguide 260 to be open at one or bothends, since the distance to the patient tunnel 16 means that theexponential attenuation is already strong enough to adhere to the limitvalues allowed in the ISM band.

It is also conceivable for the MRI scanner 1 to be surrounded byscreening that has larger dimensions than the half wavelength. Thenhowever, instead of a radio-frequency-tight door, a tunnel-shaped accessopening 261 made of conducting material with a correspondingly smallcross section to the MRI scanner 1 can be provided, the dimensions ofwhich suppress the free propagation of the wave by a cut-off frequencygreater than the Larmor frequency. The access opening 261 in this caseis preferably connected electrically-conductively for radio frequencywith the screening and/or the waveguide. In a form of embodiment thewaveguide 260 is also connected electrically-conductively to the patienttunnel 16 for radio frequency.

FIG. 6 shows a schematic diagram of a form of embodiment of an inventiveMRI scanner 1 with an interference suppression transmitter 80.Electrical waves or alternating waves are also able to be suppressed byelectrical fields with the same frequency and amplitude amount, butopposite polarity or a phase displacement by 180 degrees. If amplitudeamounts or phase do not match exactly, then at least a reduction by thedestructive interference is achieved. An inventive MRI scanner 1 hasinterference suppression antennas 81 for creating these alternatingfields for interference suppression, which are arranged around thesource of the fields, here around the patient tunnel 16. Preferably theinterference suppression antennas 81 cover all spatial directions aroundthe opening and a symmetry is used, such as e.g. the same distances tothe opening of the patient tunnel 16 and/or a distribution at the sameangular distances to the opening in order to simplify an activation ofthe individual interference suppression antennas 81. Through amplitudeand phase able to be set individually for each interference suppressionantenna 81, however, any given distribution is also conceivable.Depending on the type of the alternating field, antennas with preferablyan electrical field such as for example dipoles or with magnetic fieldsuch as for example transmit coils can also be involved here. Thealignment of the antennas or the polarization of the field generated inthis case is preferably oriented to the field direction of thealternating fields to be suppressed.

The signal that is transmitted from the interference suppressionantennas 81 is intended to reduce the emission of the excitation pulseand must thus have a predetermined amplitude and phase relationship tothe excitation pulse. Preferably the signals are therefore derived asanalog signals or also from the digital pulse generation. It is howeveralso conceivable to provide the signals through separate unitsindependently of the pulse generation, provided the necessary amplitudeand phase relationship is established.

In FIG. 6, a connecting line between the body coil 14 as source of theelectromagnetic waves and the interference suppression transmitter isspecified symbolically. A direct connection via a power distributor orfor example a directional coupler would be conceivable, a sensor in thepatient tunnel would also be possible for direct acquisition of theelectromagnetic field. It would also be possible however to obtain areference signal for creating the signal for interference suppressionfrom the power amplifier 222 or the pulse generator 220.

The reference signal for interference suppression derived from theexcitation pulse is subsequently delayed or phase-shifted by adjustablephase shifters 82 for the individual interference suppression antennas81 and subsequently has its amplitude amplified by adjustable amplifiers83, before it is transmitted via the interference suppression antennas81.

The phase shifters 82 and the amplifiers 83 are adjusted in this case byan interference suppression controller 84 via a signal connection. It isconceivable in this case for the interference suppression controller 84to set predetermined phase shifts and amplitudes, which are establishedfor example during the installation of the MRI scanner 1.

It is also possible however for the adjustment to take place by acalibration measurement. In this case it is conceivable for acalibration receiver 85 to record the alternating field to be suppressedby means of one or preferably more calibration elements 86 distributedin the room. At the same time the calibration receiver 85 acquires thesignals supplied to the interference suppression antennas 81 andtransfers the acquired values to the interference suppression controller84. The interference suppression controller 84 can then for exampleadjust the interference suppression controller 84 by a linearoptimization method such as LSR of the phases and amplitudes of theindividual interference suppression antennas in such a way that, at thelocation of the calibration antennas 86 the field strength is zero. Ifthe n calibration elements 86 are distributed across the spatial anglethen the resulting alternating field from body coil 14 and interferencesuppression antennas 81 can be changed to a multipole field with n zeropoints or radiation areas, which decrease at a higher power withdistance and make an effective suppression possible.

Basically in this case the propagation of the fields is reversible. Forcalibration it would also be conceivable for the calibration element orelements 86 to transmit a signal and for the body coil 14 and theinterference suppression antennas 84 to receive the signal and then forthe interference suppression controller 84 to establish a suitable phaserelationship and amplitudes.

Moreover, the calibration element 86 could also be used for transmittinga reference signal for receive interference suppression. In this casethe reference signal would have to be encoded or modulated so that it isable to be distinguished from a magnetic resonance signal by thereceiver 70. This could for example also be achieved below theinterference limit of the magnetic resonance signal with aspread-spectrum modulation. Also conceivable would be a transmission inan adjacent frequency range. In this case it is necessary for thereceiver 70 to be able to establish a correlation between the referencesignal and the signals received via the second and first receivingantennas, in order to optimize the interference suppression. In this waysettings for suppression of interference signals from specificdirections could be determined for example.

FIG. 7 shows a schematic of a flowchart of a possible form of embodimentof an inventive method for operation of the inventive method. In FIG. 7,the aspect of how an excitation pulse has to be designed and transmittedis considered in particular, in order to adhere to regulatory limitvalues for radio frequency radiation even without a screening cabin, inparticular when the Larmor frequency lies in an ISM band. The steps ofan inventive method already explained in FIG. 3 are summarized in FIG. 7under step S130 and are not explained once again. However it isbasically also possible to implement the measures explained for FIG. 7for limiting emission during the excitation pulse even without thereceive interference suppression from FIG. 3.

In a step S110 an excitation pulse for excitation of nuclear spins in anexamination object is determined by the controller 23. To do this, firstof all, in a step S111 an excitation pulse for excitation of the nuclearspins in a slice of the examination object is established by thecontroller 23. This can take place for example as a function of theselected sequence or type of examination by selecting from a library ofexcitation pulses. The frequency, the duration, the power and thespectral distribution in this case depend on a number of parameters. Themid frequency is produced from the nuclear spins to be acquired, thestrength of the homogeneous static magnetic field BO, the location ofthe slice in relation to the gradient field and also the strength of thegradient field. The spectral distribution and bandwidth for its part isproduced from the strength of the gradient field and the thickness ofthe slice in the direction of the field gradients. The amplitude for itspart depends on the duration of the excitation pulse, the volume to beexcited and the desired excitation strength, also referred to as theflip angle. In this case, in sub-step S111, a set of parameters isdetermined as a function of these boundary conditions, which describe apossible excitation pulse for these boundary conditions. It isconceivable for a library or table of different sets of parameters to bespecified for specific standard situations, such as for exampleacquiring images of specific organs, and to be selected from it.

In a further sub-step S112 a check is performed by the controller 23 asto whether the established excitation pulse lies within thepredetermined frequency limits. In the simplest case the highest and thelowest frequency of the excitation pulse can be computed for examplewith the aid of the mid frequency and the spectral frequencydistribution. It is also conceivable in this case to compute the powerdistribution and assess limit values for a frequency-dependent power.

The sub-step S111 is repeated if it is determined in the assessment thatthe established excitation pulse exceeds limit values, in particularlimit values for an allowable emission of radio frequency power. Thisrelates above all during operation of the MRI scanner in an ISM band toan emission outside the ISM band that is subject to greaterrestrictions.

In this case, parameters that influence this are varied on repetition ofsub-step S111. A longer pulse can achieve the same excitation with alower power for example. With a smaller gradient a smaller frequencybandwidth is required in order to excite the same slice thickness.

If the excitation pulse established in sub-step S111 adheres to thelimit values, then it is transmitted in a step S120 of the method by theradio frequency unit 22.

In a step S130, as already described for FIG. 3 in an exemplary form ofembodiment, the magnetic resonance signal is received by the receiver70.

Subsequently, in a step S140, a map of a distribution of nuclear spinsby the controller 23 from the received magnetic resonance signal isestablished. Preferably the mapping is finally reproduced on a display.

FIG. 8 shows a schematic of a flowchart of a further sub aspect of theinventive method shown in FIG. 3, here a possible interferencesuppression by previously acquired image information.

In this case the receiver 70 receives in a step S21 a first receivedmagnetic resonance signal and stores it in a memory. In this case it isalso conceivable for the signal acquired in step S21 to originate from acalibration measurement or a pre-scan that has also been acquired withother parameters or a lower resolution.

In a further step S22 the receiver 70 receives a second receivedmagnetic resonance signal and stores this. Preferably the secondmagnetic resonance signal involves a signal for an image acquisition.

In a step S23 the first received magnetic resonance signal and thesecond received magnetic resonance signal are compared. This can alreadytake place in the raw data for example or not until the image space, forexample after a Fourier transform. In a preferred form of embodiment thecomparison is undertaken on a row basis in the k-space. If the firstmagnetic resonance signal and the second magnetic resonance signaldiffer significantly, in particular if a possibly different recordingsituation has already been taken into consideration in the comparison,then with a deviation that is to be attributed to external interferencean interference suppression measure is performed. Interference signalscan be characterized by the frequency, amplitude or a characteristiccurve or duration for example.

An interference suppression measure in this case can be a repetition ofthe acquisition for example that, in particular for a row in thek-space, leads to shorter delays. It is also conceivable to set thesignal to zero, above all if it should involve a region from which noimage signal is to be expected.

FIG. 9 shows a schematic of a flowchart of a further sub aspect of aninventive method for interference suppression, here a possibleinterference suppression by analysis of the acquired image informationin an image space.

In this possible form of embodiment of the inventive method, themagnetic resonance signal is investigated in the image space to detectthe interference signal and to define the parameters for interferencesuppression. To do this, in a sub-step S42, the received magneticresonance signals are transformed by the controller 23 in an imagespace, for example by a Fourier transform. This step can also beperformed like the subsequent steps on individual rows of the raw dataspace, so that a detection and correction can be performed more quickly.

In another sub-step S43, the interference signals are separated by thecontroller 23 from the magnetic resonance data. This is possible forexample if regions in the image space can be determined by asegmentation from a pre-scan or other additional information aboutpatient and location, at which no magnetic resonance signals fromnuclear spins are to be expected. Signals arising there in the imagespace are then to be assigned to a disruption.

In a further sub-step S44, the interference signals are transformed backinto a raw data space, for example by a further Fourier transform.

In another sub-step S45, the parameters for interference suppression canthen be determined from the interference signals separated from theuseful signal and transformed back in the raw data space, for example asphase and amplitude for destructive interference in the receiver fromthe signals of the first and second receiving antennas. It is alsoconceivable in this case for the steps of back transformation and thedetermination of parameters to be linked to one another, since frequencyand phase in the raw data space are linked to the position in the imagespace.

FIG. 10 shows a schematic of a flowchart of a further sub aspect of aninventive method for interference suppression, which establishestransfer functions between first receiving antennas and second receivingantennas 60 with one or more additional calibration elements 86.

In a sub-step S80 in this case a transfer function between a firstreceiving antenna and the calibration element 86 is established. Forthis it is conceivable for a signal to be transmitted by theinterference suppression controller via the calibration element 86 withthe coordination of controller 23, which is received and evaluated bythe first receiving antenna. Preferably the signal is encoded by meansof pseudo random sequence or in another way in such a way that acorrelation between sent and received signal can be easily establishedby the receiver 70.

In a sub-step S82 a transfer function between a first receiving antennaand the calibration element 86 is established in the same way. For thisit is conceivable for a signal to be transmitted via the calibrationelement 86 with the coordination of the controller 23 by theinterference suppression controller, which is received by the secondreceiving antenna 60 and is evaluated in the receiver 70.

On account of the reversibility of the propagation of theelectromagnetic fields signals could also be sent however by the firstreceiving antenna and the second receiving antenna 62, which will bereceived by the calibration element 86.

In another sub-step S82 at least one parameter for interferencesuppression as a function of the measured transfer functions is set insuch a way that a portion of an interference signal received by thesecond receiving antenna 60 is reduced in a signal received by thereceiver 70 via the first receiving antenna. For example it can bedefined in each case via the transfer functions how an interferencesignal arrives from the direction of the calibration element 86 at theinput of the receiver 70 via the first receiving antenna and the secondreceiving antenna 60, in particular with which amplitude and phasedisplacement. Thus for example an additional phase displacement can beset in receiver 70, so that the signals from first receiving antenna andsecond receiving antenna are superimposed destructively in the receiverand the fault is suppressed. As a further parameter, the amplificationof the amplitude can be set so that there is an extinction for aninterference signal from a point in space. With a number of firstreceiving antennas and second receiving antennas 60 more parameters orpairs of parameters are to be adapted accordingly, which can take placefor example by linear optimization methods such as LSR.

Shown in FIG. 11 is an interaction between a number of inventive MRIscanners 1. In this case the control unit 20 of the first MRI scanner 1receives a signal from a second MRI scanner 101 via an interface. Thecontrol unit 21 in this case is designed to synchronize an imageacquisition as a function of a signal received via the interface fromthe second MRI scanner.

A number of possible forms of embodiment are shown simultaneously inFIG. 11. On the one hand the interface can be the LAN interface 26, viawhich the MRI scanner 1 is connected to the second MRI scanner 101 forsignaling. Also conceivable however are all other interfaces for theexchange of information such as WIFI, WAN or serial or parallelpoint-to-point-connections.

In this case it is conceivable in a form of embodiment for the secondMRI scanner 101 to transmit a message about a planned image acquisitionwith the signal. The message can for example specify that at a specifictime t, at the frequency f, an excitation pulse of duration d will besent by the second MRI scanner 101. The control unit 20 of the first MRIscanner 1 then synchronizes its own image acquisition as a function ofthis information.

One possibility is for the control unit 20 to synchronize its ownexcitation pulse in such a way that it occurs at the same time, sincebecause of the extremely high field strengths that are required forexcitation, the excitation pulses of adjacent MRI scanners, because ofthe attenuation already provided by the construction of the MRIscanners, do not disrupt one another.

More sensitive to disruptions on the other hand is the receipt ofmagnetic resonance signals from the examination volume or the patient100. Since here an attenuation in relation to the excitation pulse ofover 100 dB is present, an excitation pulse of an adjacent MRI scanner101 can disrupt the receipt of MR signals even when screening ispresent. The control unit 20 of the MRI scanner 1 can therefore plan andcarry out the image acquisition in such a way that this does notcoincide with the excitation pulse of the second MRI scanner 101. Forexample own excitation pulses and the readout sequences dependentthereon can be applied so that the receipt windows of the first MRIscanner 1 do not coincide with the excitation pulses of the second MRIscanner 101.

In this case it is also conversely possible for second MRI scanner 101to transmit information about a planned receipt. The message can forexample specify that at a specific time t, at the frequency f for theduration d, an MR signal is to be recorded by the second MRI scanner101. The first MRI scanner 1 can then set its own transmission processso that no transmission process takes place in the time window specifiedin the message, at least not on a frequency band that comprises thefrequency f including a bandwidth specified in the message.

Finally combined messages are moreover conceivable, in which transmitand receive processes are agreed mutually between the first MRI scanner1 and the second MRI scanner 101, preferably in such a way that theimage acquisition devices can be executed with the least possible delayby interleaving.

In another form of embodiment, which is shown in FIG. 11, it is alsoconceivable however for the signal to be a radio wave of an excitationpulse itself and the interface for example the local coil 50 with theradio frequency unit 22. Preferably in this case receipt also or justtakes place at times at which the first MRI scanner 1 is not itselfrecording an MR signal. On the basis of the excitation pulse the controlunit 20 can detect that a second MRI scanner 101 has just transmitted anexcitation pulse and therefore is then planning an acquisition of amagnetic resonance. It is then for example conceivable for the first MRIscanner 1 then not to transmit any excitation pulse itself for a certaintime. It would also be possible for the first MRI scanner 1 to use theexcitation pulse of the second MRI scanner 101 itself as a trigger pulseand to transmit its own excitation pulse almost synchronously, sinceusually pauses without receipt and thus possible mutual interference liebetween excitation pulse and receipt of the magnetic resonance signal.

Regardless of whether the transmission of an excitation pulse isdetected directly via the received electromagnetic field of the pulse orvia a message via the data interface, it is also conceivable in thiscase for the control unit 20 to change the frequency of the nextexcitation pulse as a function of the signal. In MRI scanning individualslices in the direction of the BO field, usually along the z-axis 2, aredifferentiated by a superimposed gradient field in the z direction intheir frequency and are thus able to be distinguished. The control unit20 can for example change the order of the scanning of individualslices, so that the first MRI scanner 1 and the second MRI scanner 101each acquire slices with different mid frequency and in this waycrosstalk or interaction are avoided by the different frequencies. Anadditional degree of freedom that the control unit 20 can use in thiscase is also the location of the patient 100 on the movable patientcouch 30 relative to the isocenter of the field magnet 10. By moving thepatient 100 along the z-axis a little, the different location inrelation to the z gradient field means that the Larmor frequency alsochanges for a slice. The first MRI scanner 1 can thus, by a relativemovement of the patient along the z-axis, also acquire the same slice inthe body of the patient 100 with different frequencies, so that aninteraction with the second MRI scanner 101 can be avoided.

Although the invention has been illustrated in greater detail by thepreferred exemplary embodiment, the invention is not restricted by thedisclosed examples and other variations can be derived herefrom by theperson skilled in the art, without departing from the scope ofprotection of the invention.

1. An MRI scanner, wherein the MRI scanner (1) has a patient tunnel(16), a first receiving antenna for receiving a magnetic resonancesignal from a patient (100) in the patient tunnel (16), a secondreceiving antenna (60) for receiving a signal having the Larmorfrequency of the magnetic resonance signal, and a receiver (70), whereinthe second receiving antenna (60) is arranged outside or in the vicinityof an opening of the patient tunnel (16), wherein the receiver (70) isconnected to the first receiving antenna and the second receivingantenna (60) for signaling and the receiver (70) is designed to suppressan interference signal received with the second receiving antenna (60)in a magnetic resonance signal received by the first receiving antenna.2. An MRI scanner, wherein the MRI scanner (1) has a patient tunnel(16), a first receiving antenna for receiving a magnetic resonancesignal from a patient (100) in the patient tunnel (16), a secondreceiving antenna (60) for receiving a signal close to the Larmorfrequency of the magnetic resonance signal and a receiver (70), whereinthe second receiving antenna (60) is arranged outside or in the vicinityof an opening of the patient tunnel (16), wherein the receiver (70) isconnected to the first receiving antenna and the second receivingantenna (60) for signaling and the receiver (70) is designed to suppressa wideband interference signal received with the second receivingantenna (60) outside a frequency range of the magnetic resonance signalin a magnetic resonance signal received by the first receiving antenna.3. The MRI scanner as claimed in claim 1 or 2, wherein the MRI scanner(1) is designed to receive magnetic resonance signals having a Larmorfrequency in an industrial band.
 4. The MRI scanner as claimed in claim3, wherein the MRI scanner (1) has a transmit path for transmitting anexcitation pulse with an ISM filter (223), wherein the ISM filter (223)is designed to suppress signals outside the ISM band.
 5. The MRI scanneras claimed in claim 4, wherein the MRI scanner has a transmit antennafor transmitting an excitation pulse, wherein the MRI scanner (1) hasnon-linear components for tuning the transmit antenna, wherein thenon-linear components are arranged in an area of the MRI scannerscreened off for radio frequency from the patient tunnel, wherein theISM filter (223) is arranged between non-linear component and antenna.6. The MRI scanner as claimed in claim 3 with a radio frequency unit(22), wherein the radio frequency unit (22) has a preliminaryinterference suppressor (221), which is designed for preliminaryinterference suppression of an excitation pulse for excitation ofnuclear spins in such a way that signal portions of the excitation pulseoutside the ISM band are reduced compared to an excitation pulse withoutpreliminary interference suppression.
 7. The MRI scanner as claimed inone of the preceding claims, wherein a limit frequency for a propagationof a radio wave in the patient tunnel (16) is greater than a Larmorfrequency of the MRI scanner (1).
 8. The MRI scanner as claimed in oneof the preceding claims, wherein the MRI scanner (1) has a waveguide(260) that surrounds the MRI scanner (1), wherein the waveguide (260)has a limit frequency that is greater than the Larmor frequency of theMRI scanner (1).
 9. The MRI scanner as claimed in claim 8, wherein thewaveguide (260) has an electrically conductive connection (262) to thepatient tunnel (16).
 10. The MRI scanner as claimed in one of thepreceding claims, wherein the second receiving antenna (60) as arrangedon an opening of the patient tunnel (16) or on the patient couch (30).11. The MRI scanner as claimed in one of the preceding claims, whereinthe second receiving antenna (60) has an omnidirectional receivecharacteristic.
 12. The MRI scanner as claimed in one of the precedingclaims, wherein the MRI scanner (1) has a plurality of second receivingantennas (60) and the receiver (70) is designed to suppress theinterference signal in the magnetic resonance signal as a function ofreceive signals of the plurality of second receiving antennas (60). 13.The MRI scanner as claimed in claim 12, wherein the plurality of secondreceiving antennas (60) is arranged in an arrangement symmetrical to thepatient tunnel (16).
 14. The MRI scanner as claimed in one of thepreceding claims, wherein the receiver (70) has an autocorrelationdevice and the autocorrelation device is designed to determine a portionof the signal received by the second receiving antenna (60) in themagnetic resonance signal received by the first receiving antenna. 15.The MRI scanner as claimed in one of claims 1 to 13, wherein thereceiver (70) has an estimation device and the estimation device isdesigned to estimate a portion of the signal received by the secondreceiving antenna (60) in the magnetic resonance signal received by thefirst receiving antenna.
 16. The MRI scanner as claimed in one of thepreceding claims, wherein the MRI scanner (1) has an interferencesuppression transmitter (80) and an interference suppression antenna(81), wherein the interference suppression antenna (81) is arranged at adistance from the patient tunnel (16), wherein the interferencesuppression transmitter (80) is designed to output a signal in afrequency range of an excitation pulse of the MRI scanner (1) via theinterference suppression antenna (81) as a function of a transmitinterference suppression parameter so that, in a predetermined area ofan environment of the MRI scanner (1), a field strength of theexcitation pulse is reduced by destructive interference.
 17. The MRIscanner as claimed in claim 16, wherein the MRI scanner (1) has acalibration element (86) in an environment of the MRI scanner (1) and aninterference suppression controller (84), wherein the interferencesuppression controller (84) is designed to detect a field strength in afrequency range of an excitation pulse at the location of thecalibration element (86) by means of the calibration element (86) and asa function of the field strength detected to set the transmitinterference suppression parameter in such a way that a field strengthof the excitation pulse is reduced in a predetermined environment of thecalibration element (86).
 18. The MRI scanner as claimed in claim 16 or17, wherein the interference suppression transmitter (80) is designed togenerate the signals for the interference suppression antenna (81) orinterference suppression antennas (81) by phase displacement and/oramplitude adaptation as a function of one or more transmit interferencesuppression parameters.
 19. The MRI scanner as claimed in one of claims16 to 18, wherein the interference suppression antenna (81) has a radiofrequency power amplifier.
 20. The MRI scanner as claimed in one of thepreceding claims, wherein the MRI scanner (1) has a calibration element(86) in an environment of the MRI scanner (1), wherein the receiver (70)is designed to measure a first transfer function between the firstreceiving antenna and the calibration element (86) and also a secondtransfer function between the second receiving antenna (60) and thecalibration element (86) and, as a function of the measured firsttransfer function and the second transfer functions, to set theinterference suppression parameter or parameters in such a way that aninterference signal received with the second receiving antenna (60) isreduced in a magnetic resonance signal received by the first receivingantenna.
 21. A method for operation of an MRI scanner (1), wherein theMRI scanner (1) has a patient tunnel (16), a first receiving antenna forreceiving a magnetic resonance signal from a patient (100) in thepatient tunnel (16), a second receiving antenna (60) for receiving asignal with the Larmor frequency of the magnetic resonance signal and areceiver (70), wherein the second receiving antenna (60) is arrangedoutside the patient tunnel (16) or in the vicinity of an opening of thepatient tunnel (16) and wherein the method has the steps: (S10) Receiptof an interference signal by the receiver (70) via the second receivingantenna (60); (S20) Receipt of a magnetic resonance signal by thereceiver (70) via the first receiving antenna; (S30) Processing of themagnetic resonance signal as a function of the interference signal bythe receiver (70) to a receive signal, wherein the dependency is afunction of a parameter; (S40) Setting of the parameter by the receiver(70), so that a portion of the interference signal is reduced in thereceive signal.
 22. A method for operation of an MRI scanner (1),wherein the MRI scanner (1) has a patient tunnel (16), a first receivingantenna for receiving a magnetic resonance signal from a patient (100)in the patient tunnel (16), a second receiving antenna (60) forreceiving a signal close to the Larmor frequency of the magneticresonance signal and a receiver (70), wherein the second receivingantenna (60) is arranged outside the patient tunnel (16) or in thevicinity of an opening of the patient tunnel (16) and wherein the methodhas the steps: (S10) Receipt of a frequency portion of the interferencesignal close to the Larmor frequency by the receiver (70) via the secondreceiving antenna (60); (S20) Receipt of a magnetic resonance signal bythe receiver (70) via the first receiving antenna; (S30) Processing ofthe magnetic resonance signal as a function of the frequency portion ofthe interference signal by the receiver (70) to a receive signal,wherein the dependency is a function of a parameter; (S40) Setting ofthe parameter by the receiver (70), so that a portion of theinterference signal is reduced in the receive signal.
 23. The method asclaimed in claim 21 or 22, wherein the step (S40) of setting theparameter has the step (S41) of temporal averaging with the formation ofa temporal average value as a function of the interference signal.
 24. Amethod for operation of a MRI scanner (1) as claimed in claim 20,wherein the method has the steps: (S80) Measurement of a transferfunction between a first receiving antenna and the calibration element(86); (S81) Measurement of a transfer function between second receivingantenna (60) and the calibration element (86); (S82) Setting of theinterference suppression parameter as a function of the measuredtransfer functions in such a way that a portion of an interferencesignal received by the second receiving antenna (60) is reduced in asignal received by the receiver (70) via the first receiving antenna.25. The method as claimed in claim 21 or 22, wherein the step (S10) ofreceipt of the interference signal is undertaken at a time of a sequenceat which no magnetic resonance signal for imaging is being received. 26.The method as claimed in one of claims 21 to 25, wherein the receiver(70) has a memory and the method has a step (S25) of storage, in whichthe receiver (70) stores the interference signal as well as the magneticresonance signal in the memory, wherein the step (S30) of processing isundertaken with a delay relative to the receipt of the interferencesignal and/or magnetic resonance signal.
 27. The method as claimed inone of claims 21 to 26, wherein the receiver (70) has an autocorrelationdevice and in the step (S40) of setting the parameter theautocorrelation device determines a portion of the interference signalin the magnetic resonance signal and the parameter is set as a functionof the portion of the interference signal determined.
 28. The method asclaimed in one of claims 21 to 26, wherein the receiver (70) has anestimation device and in the step (S40) of setting the parameter theestimation device determines a portion of the interference signal in themagnetic resonance signal and the parameter is set as a function of theportion of the interference signal determined.
 29. The method as claimedin one of claims 21 to 26, wherein the step (S40) of setting theparameter the has the sub-steps: (S42) Transforming the receivedmagnetic resonance signals into an image space; (S43) Separating theinterference signals from the magnetic resonance data; (S44)Transforming the interference signals into a raw data space; (S45)Determining the parameters from the transformed interference signals inthe raw data space.
 30. The method as claimed in claim 29, wherein thesteps of the transforming (S42), of separating (S43), of backtransforming (S44) and of determination of the parameter are undertaken(S45) on rows of data of the received magnetic resonance signals in theraw data space.
 31. The method as claimed in one of claims 21 to 30,wherein the receiver (70) monitors the interference signal for changesin a step (S50) and if there is a change, adapts the parameter in a step(S51).
 32. The method as claimed in one of claims 21 to 31, wherein thereceiver (70) stores a first received magnetic resonance signal in amemory in a step (S21); stores a second received magnetic resonancesignal in a step (S22) and compares the first received magneticresonance signal and the second received magnetic resonance signal in astep (S23) and if there is a deviation which is attributable to externalinterference, performs an interference suppression measure.
 33. Themethod as claimed in claim 32, wherein the interference suppressionmeasure is a discarding (S60) of the first and/or second receivedmagnetic resonance signal, a repetition of the acquisition of the firstand/or second magnetic resonance signal or setting (S40) of theparameter.
 34. A method for operation of an MRI scanner (1), wherein theMRI scanner (1) has a patient tunnel (16), a first receiving antenna forreceiving a magnetic resonance signal from a patient (100) in thepatient tunnel (16), a second receiving antenna (60) for receiving asignal having the Larmor frequency of the magnetic resonance signal anda receiver (70), wherein the second receiving antenna (60) is arrangedoutside the patient tunnel (16) or in the vicinity of an opening of thepatient tunnel (16) and wherein the method has the steps: (S10) Receiptof an interference signal by the receiver (70) via the second receivingantenna (60); (S20) Receipt of a magnetic resonance signal by thereceiver (70) via the first receiving antenna; (S60) Discarding of themagnetic resonance signal as a function of the interference signalreceived by the second receiving antenna (60).
 35. A method foroperation of an MRI scanner (1) having a Larmor frequency in an ISMband, wherein the method has the steps: (S110) Determination of anexcitation pulse for excitation of nuclear spins in an examinationobject; (S120) Transmission of the excitation pulse; (S130) Receiving amagnetic resonance signal; (S140) Determination of a mapping of adistribution of nuclear spins in the examination object; wherein, in thestep (S110) of determination of the excitation pulse, the pulse isdetermined as a function of predetermined frequency limits of the ISMband.
 36. The method as claimed in claim 35, wherein the step (S110) ofdetermination of an excitation pulse has the sub-steps: (S111)Establishing of an excitation pulse for exciting nuclear spins in aslice of the examination object as a function of a position of the slicerelative to a magnet unit (10), a predetermined gradient strength, athickness of the slice and the type of measurement; (S112) Checkingwhether the established excitation pulse lies within the predeterminedfrequency limits of the ISM band; Repetition of the step (S111) ofestablishing while varying a pulse parameter, which on establishing ofthe excitation pulse has an effect on a spectral frequency distributionof the excitation pulse if the excitation pulse does not lie within thepredetermined frequency limits or Transmission of the establishedexcitation pulse in step (S120).
 37. The method as claimed in claim 36,wherein the pulse parameter that affects the establishing of theexcitation pulse is one of the parameters duration of the excitationpulse, thickness of the slice, relative position of the slice orstrength of the gradients.
 38. The method as claimed in claim 35,wherein the step (S120) of transmission has the sub-step (S121) ofchanging the position of the examination object relative to the magnetunit (10) before the step (S122) of emitting the pulse.
 39. The MRIscanner as claimed in one of claims 1 to 20 with a control unit (20) forcontrolling the image acquisition and an interface connected to thecontrol unit (20) for signaling, wherein the control unit (20) isdesigned to synchronize an image acquisition as a function of a signalreceived via the interface from a second MRI scanner (101).
 40. The MRIscanner as claimed in one of claims 1 to 20 with a control unit (20) forcontrolling the image acquisition and an interface connected to thecontrol unit (20) for signaling, wherein the control unit (20) isdesigned to transmit a signal with information about an impending imageacquisition to a second MRI scanner (101).
 41. The MRI scanner asclaimed in claim 39 or 40, wherein the interface is designed for theexchange of data, wherein the control unit (20) is designed, by means ofinformation exchanged via the interface, to synchronize an imageacquisition with a second MRI scanner (101).
 42. The MRI scanner asclaimed in one of claim 40 or 41, wherein the signal has informationabout a time and/or frequency of a transmission process.
 43. The MRIscanner as claimed in one of claim 40 to 42, wherein the signal hasinformation about a time and/or frequency of a receive process.
 44. Acomputer program product, which is able to be loaded directly into aprocessor of a programmable controller (23), with program code means forcarrying out all steps of a method as claimed in one of claim 21 to 38when the program product is executed at the controller (23).
 45. Acomputer-readable storage medium with electronically-readable controlinformation stored thereon, which is designed in such a way that, whenthe storage medium is used in a controller (23) of an MRI scanner (1) asclaimed in claim 1 to 20, it carries out the method as claimed in one ofclaim 21 to 38.